Conway's IRIS and the windscreen wiper theorem
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- čas přidán 5. 04. 2024
- Conway's whatever ... it's named after John Conway and so it must be good :)
Wiki page dedicated to John Conway
en.wikipedia.org/wiki/John_Ho...
Wiki page Conway's circle
en.wikipedia.org/wiki/Conway_...
Wiki page on his Game of Life
en.wikipedia.org/wiki/Conway%...
Michael de Villiers (the connection with the windscreen wiper theorem, read this one first)
dynamicmathematicslearning.com...
dynamicmathematicslearning.com...
Also check out Michael's main page dynamicmathematicslearning.com...
Matt Baker's tribute blog to John Conway
mattbaker.blog/2020/04/15/som...
Tanya Khovanova's blog post on Conway's circle showing Conway's wearing a Conway circled Mathcamp t-shirt
blog.tanyakhovanova.com/2010/...
Colin Beveridge's proof without words
aperiodical.com/2020/05/the-b...
Colin Beveridge and Elizabeth A Wilson (his proof without words explained :)
aperiodical.com/2020/06/conwa...
Paul Farrel's beautiful animated proof
• John Conway Circle The...
Me on Conway's circle and Conway's pinwheel tiling for the Gathering of Gardner
• Burkard Polster - Anim...
Card Colm on Conway and his circle
www.cardcolm.org/JHC.html
cardcolm.org/Pics/RybaConwayC...
Eric Braude
Conways Circle Theorem: A Short Proof Enabling Generalization to Polygons
arxiv.org/abs/2111.01835
Mathologer video on curves of constant width
New Reuleaux Triangle Magic
• New Reuleaux Triangle ...
Mathologer videos on Conway's proof of Morley's miracle
Math is Illuminati confirmed
• Math is Illuminati con...
Part 2
• Math is Illuminati con...
Music: Rain by ANBR and Ethereal Ottom
T-shirt: ages old, don't remember where I got this one from
Enjoy!
Burkard
Conway’s circle has the larger diameter. The “diameter” of the shape of constant width is exactly one of the chords we were discussing. But in Conway’s circle, those same chords are not diameters, so are less than a diameter, so the circle has a greater diameter.
Exactly :)
Just to go a step further, the perimeter of the shape described by the wiper arcs is exactly pi times the perimeter of the triangle i.e. (pi * the chord length), whereas the circumference of the circle is (pi * 2 * the radius) and, as noted above, it is easily seen that (2 * the radius) is greater than (the chord length).
To check that the perimeter of the SOCW is (pi * the chord length), consider each arc length. If we label the triangle sides A,B,C and their opposite angles a,b,c, then the arc lengths are given by: Aa + Bb + Cc + (B+C)a + (A+C)b + (A+B)c = (A+B+C)(a+b+c) = (chord length)(pi)
Yes, you are right. My intuition was wrong. I love how these videos get people talking-any ideas on getting 15-16 year olds talking like this?
To take it one (very small step further), this shape of constant width has the smallest diameter of all shapes of constant width containing all 6 points!
@@Falanwe that suggests it might be a very valuable curve to quickly generate for certain applications, especially manufacturing and construction. Minimization is a huge part of engineering & design.
RIP conway, he will be missed
Very much so :(
He *is* missed, sorely. I'm disappointed I didn't get to know everything he'd done before he died, only knowing about his Game of Life.
@@royalninja2823he says that when he was younger, he used to get upset that people only know his game of life, because it was a quick and draft invention.
But I also only know his game of life.
Apparently the last thing he saw before he died was 3 black squares.
“Houston, they got me in a glider pattern. I’m just a dead cell now. Report: dead cell now only.”
I remember translating this theorem's Wikipedia page to Hebrew a few years ago, I came up with the coloring proof. but when I saw the swivel proof I just couldn't help but smile. I liked it better than my original proof. this proof made me smile so much. this always happens in your videos, in what you call "aha moment" I experience euphoria. your videos just make me smile from ear to ear, every one of them.
I make you smile, that's great ! :)
I liked the color proof better, because the swivel proof seemed like it relied more on visuals, which could be deceiving.
same
Colour proof seems like something I could come up with given enough time and determination (so back to school). The swinging one reminds me of 3B1B exercise of a proof easy to understand, but super hard to come up and write rigorously.
@@Filipnalepa Agreed. It was even similar in the fact of the rotation of a line being a part of it (although in this case it’s a segment rather than an actual line).
"You're here for the mysterious iris in the thumbnail, aren't you?"
Sorry, but no. I'm here because it's another Mathologer video :)
Of course that's also a reason that gets my seal of approval :)
Yes, if I have to choose, I watch the latest Mathologer video before the latest 3B1B.
I wanted to see little creatures crawling around the screen...
@@tulliusexmisc2191 Same here.
You triggered my math competition PTSD 😂
Draw the chord that joins the vertices of the two green segments (top). This gives an isosceles triangle, hence the axis of the chord is a bisector of the top angle of the original triangle. Hence this axis goes through the triangle's incenter, and this incenter is also equidistant from the two origial top points. The same is true for all three pairs of points. Now choose the top right and the most right point: both lie on a (green+blue) segment. Reason just the same as before and you get that these two points also are equidistant from the triangle's incenter. Rinse and repeat, and you get that the incenter is equidistant from all six points.
My first thoughts as well! Go team bisectors!
yes, this is the proof i got also.
I did it with mirrors, but I found (and posted) essentially the same proof. But you beat me to it, congrats.
Iris-free Proof: join the six exterior points consecutively, calling them A' A'' B' B'' C' and C''. Note the numerous isosceles triangles in this diagram and mark the relevant congruent pairs of base angles. Next, observe that A'A''C'C'' is a quadrilateral with supplementary opposite angles. Thus it is a cyclic quadrilateral and A'A''C'C'' (circle 1) are concyclic. The same reasoning shows the sets of points A''B'B''C' (circle 2) and B'B''C'C'' (circle 3) are concyclic. Circle 2 and 3 have three points in common, and so A''B'B''C'C'' are all concyclic, since three non-collinear points determines a circle. This circle has three points in common with circle 1, and so A'A''B'B''C'C'' are all concyclic.
I am not sure we were thinking about the same thing but one thing that first crossed my mind seeing all the equal chords was that if i pick any 2 inersecting ones and unite the points they should form up a isosceles trapesoid which is insciptible in a circle. And indeed it is easy to prove the 2 bases are parallels as bases of isosceles opposite triangles and the sides are identical by tringle equalities. Then we just repeat for the other pairs of points and we note that circle of the circle will be somewhere on mediator of bases which also is the mediator of original triangle sides.
Puzzle at the end: Conway's circle is larger. The weird curve's diameter is precisely the length of the chords since the chords are perpendicular to the curve's tangent at the intersection (this can be trivially shown from the curve's construction), so the diameter of the circle is necessarily larger (specifically, the square of the diameter of Conway's circle is the square of the incircle's diameter plus the square of the sum of the three sides of the triangle).
Thank you for all the effort you put into these videos. I very much enjoy the learning experience from watching this channel!
I like the coloring proof best. Conway left us so many gems.
colouring proof is the one i came up with after you pointed out the incircle. also, the shape of constant width has width equal to one of the chords of the iris, definitely less than the diameter of the circle.
Correct. I guess you are ready for when you meet Conway in the afterlife :)
This was heartachingly beautiful. Thank you.
Kudos! To think that such simple principles could go unnoticed for so long. Which is why I love this channel, come to think of it. Not only is the content absolutely amazing, I always feel like I've learned something new. (Almost as if taking a glimpse into the realm of "sacred geometry" or something!)
That's great :)
The iris was interesting, but I'm here for Conway!
Definitely the better reason :)
Came for a fractal that looks like an iris, stayed for the beautiful visual geometric proofs
I liked the colored line proof the best. The other one seems like something someone would show you and it would not turn out to not be true, but just looks like it.
Love your channel!
Wonderful proof. The incentre is slowly becoming my favourite triangle centre! (overtaking the orthocentre as my previous favourite)
Weird curve's diameter= L1+L2+L3 which is a chord of Conway's circle not containing its center < diameter of Conway's circle.
BTW, (Conway's Radius)^2 = r^2 + p^2, here r is the incenter's radius and p is both the triangle's semiperimeter and the weird curve's "radius" (there's no center).
You can get r = H/p, using Heron's Formula (H)
Correct :) Here is a little a little trickier question than. Which is larger the AREA of Conway's circle or that of the curve of constant width?
The area of Conway’s circle is larger. I think the proof is quite complicated to write down, so I don’t think I can do this. But I can conceptualize the proof like this:
There are 3 pairs of fan-shaped sectors that, when combined and subtracting twice the area of the inner triangle, become the area of The Weird Curve.
We can choose one pair of large and small fan-shaped sectors opposite each other to prove this, and the other two pairs will be proven similarly.
We see that the small fan-shaped sector will always extend beyond the Conway circle, and the large fan-shaped sector will always eat inside the Conway circle.
Using the formula to calculate the area of a circular arc, we can prove that the area extending outside the Conway circle of the small fan-shaped sector is always smaller than the area eating inside the Conway circle of the large fan-shaped sector opposite it
Thanks for the video! I felt compelled to try to find a proof, so here it is. The center of the big circle must be the intersection of the three lines cutting the angles of the triangle in two. This is also the center of a small circle, inscribed in the triangle. Consider a segment of length equal to the sum of the three sizes of the triangle. If we glue the middle of this segment to side of the small circle, and rotate this construction, the extremities of the segment draw the big circle (I skipped a few details that were a pain to write concisely : ) Thank you for making me feel worthy! I hope I got it right
I've only watched 45 seconds of this video, so far. And I have to say... "Wow!" A circle... Wow. Greetings from Dallas, Texas!
Greetings from Melbourne in Australia :)
Spoiler alert:
Irving here...
I know "Numberphile" might be a complicated word around these parts.. but i will never forget watching that interview Brady did with him [Conway] when he's looking out the window and says "i wish i knew whyyy..." .. in reference to the strange universe-implying numbers in group theory - why the monster and no more?.. why any of the sporadjcs?.. especially when symmetry seems to play such a big role in fundamental physics
I have no issues with Numberphile except for a handful of videos. I definitely have no issues with their Conway videos :)
@@Mathologer But I thought 1+2+3+…=-1/12 was incontrovertible truth?
;)
@Mathologer my dad, my son and I love your vids so much. It's always a treat when a new one pops up. Thanks for doing what you do!
focus on one intersection of two supposed chords and notice that it has the same power wrt one pair of endpoints as the other (the colors show it) and this means the ends of those two chords lie on the same circle, now if we show that an endpoint of horizontally oriented chord lies on the smame circle as the circle formed by endpoints of other two chords, we are done. Label P,Q;R,S;U,V the endpoints in the positive angular direction starting with horizontal red endpoint and A the red vertex of triangle, B blue vertex and C green vertex. Since PBV is isosceles, angle
7:39 the coloring proof I suppose. The static picture that you can look at and check over feels more comforting somehow.
Great as always
To prove this, consider two intersecting chords. If you establish that the product of the segments created by their intersection point is equal for both chords, then they are points on a circle. This is evident from the equation r(b+g)=r(g+b), indicating they lie on a circle. To further prove that this circle shares the same center as the inner circle, examine, for example, a red isosceles triangle. The center of the larger circle lies on the bisector of these red arms. This bisector also serves as the bisector of two sides of the circle where the center of the inner circle lies, confirming they share the same center.
2:12 Consider the vertex of the triangle at the bottom-left. There are two lines going through it, both with red on one side and green+blue on the other side. Thus, the angle bisector of these two lines consists of all points that are equidistant from the red endpoints, _and_ (separately) equidistant from the green+blue endpoints. But applying the same reasoning to the other vertices, this leads us to the intersection of all three angle bisectors of the triangle, namely the incenter. We have six equalities between the distances from the incenter to the endpoints, which combine to show that they are all the same! q.e.d.
I guess you are ready for an encounter with Conway in the afterlife :)
Very interesting! This reminds me of a result I came across recently. Take a regular polygon positioned anywhere in a circle. Extend the sides to form two sets of segments with lengths x_1,x_2,...,x_n and y_1,y_2,...,y_n, so that each y_i is counterclockwise from x_i. Then applying power of point to each vertex, we get
x_k(L+y_(k+1))=y_k(L+x_(k-1)),
where L is the side length of the polygon. Adding all these equations and cancelling the x_ky_(k+1) terms, we get
x_1+x_2+...+x_n=y_1+y_2+...+y_n.
So the two groups of segments have equal sums.
Where did you come across this?
@@Mathologer I originally came up with the problem myself, but I later found a special case of it on the 1989 All-Russian Math Olympiad. That contest used an equilateral pentagon instead of an equilateral n-gon.
@@MathFromAlphaToOmega I see, thanks for sharing this with me :)
The "windscreen wiper" method can be used to prove the same result on a sphere, which by extension also proves the 2D case as well.
1)Draw a great circle around a sphere, marking two points at random on this circle.
2) Mark a third point at random, but not on the same great circle, and connect this to the first two points around two new great circles. This creates a spherical triangle.
3)Rotate the sphere so that the "north pole" lies within the triangle in such a way that the northernmost points on the three great circles all lie at the same latitude.
4)Draw the "whiskers" by extending the sides of the triangle as shown in the video, following the great circles in each case.
5)The great circles all have exactly the same radius of curvature at every point, so the windscreen wiper method can be used to rotate and tilt any one of the extended sides around each vertex to bring it into superposition over the adjacent side. Three rotation and tilts brings a side back to its original position, but pointing the opposite direction, just as in the video. This proves that the three extended sides are the same length, and the northernmost points are the exact midpoints of each arc.
6)Starting at the northernmost point of any of the three great circles and measuring the same arc in each direction takes you to two points which lie at the same latitude. Since the three arcs are all the same length, all six end points must lie at the same latitude.
Ta dah.
7)Let the radius of the sphere tend to infinity. The surface of the sphere tends to a plane surface, and the original Euclidean result follows directly.
Ta dah!
Sounds right :)
If this is correct, then something similar might be expected also to work in hyperbolic surfaces of constant curvature.
The swivel approach resonated better with me.
Wooo I realized this is a new Mathologer video after watching it! 🙂
Great names of the theorems and this is the best recreational math channel!
Glad you liked it!
The last curve is making nodes and antinodes with the circle.
Hello Mathologer!
You again created an amazing video about an interesting geometric theorem and made the proof visible for us.
Magic! -
Thanks, Greetings and best wishes!
Glad you liked it :)
A beautifully done video & presentation! Thank you for the mention too! 😉
Thanks again for your help answering my questions :)
That is the most fantastic Mathologer I've ever seen 🤩. My mind is spinning ! I will surely dream of this - hoping it won't transform in a nightmare😵💫
Coloring proof!
Beautiful
Love this. Thank you.
Glad you enjoyed it!
Also this shows a general method for generating curves of constant width. Given a set of strait lines of general position (no two lines are parallel, no three lines intersect at the same point) you can always draw a constant width curve as complex as you want. I don't remember the algorithm exactly but I suppose you'll manage to reconstruct it.
Very inspiring, thanks!
👍🏻 Another great T-shirt. Of course the lecture too. 😉
Thank you!
I really enjoyed this one, John Conway did so much cool stuff.
RIP
I have proven in my head... It just creates bunch of isosceles triangles, so it is easy to prove then that for every quadrilateral that you can make out of 4 consequtive points opposite angles add to 180... So all of them of them are cyclic. And as all adjecent ones share 3 points and those 3 points define a circle all six are on a same circle.
The constant width shape of your video has 6 points, but in your previous video about constant width shapes, they all had an odd number of sides. Is there a way to generalize this to create more even sided shapes of constant width ?
Each line extending from the edges of the triangle is the same length, since its length is the sum of the lengths of all three edges. The lines are also the same distance from the incenter. If you construct a line from the incenter to the closest point, and a line from the incenter to the ends of the lines, it will form a right triangle. Then you can calculate the distance from the ends of the lines to the incenter. If the closest point on the line exactly divides it in half, then the distance from the incenter to every point is the same.
Labeling the endpoints CW from top left: A, B, C, D, E, F
Any circle through A and B is centered somewhere on the angle bisector of the top angle
Any circle through B and C is centered somewhere on the angle bisector of the left angle
(similar for C-D, D-E, E-F, F-A)
QED: all the points lie on a circle centered at the intersection of the angle bisectors.
(The intersection of the angle bisectors is the center of the incircle.)
Not proved here, but true: the angle bisectors intersect at a single point.
Yes....
My favourite proof is the chord version. Even after playing around with this in Geogebra, I spent extra time with that.
Conway discovered so many things in math, he truly was a genius.
Any nice geogebra file to share ? :)
Construct the angle bisectors of the triangle. They meet at the incenter of the triangle. Draw a circle around the incenter through one of the endpoints, that are supposed to lie on a circle. Choose an angle bisector and do a reflection operation across it. The reflection maps the circle onto itself and two pairs of endpoints to each other. All endpoints can be mapped to each other by a sequence of reflections across angle bisectors. In conclusion, all these points lie on a circle. QED
Sliding the ends of a chord around the outer circle sweeps the iris (twice). The chord's center draws the inner circle (pupil).
High quality math video. The ending score makes it perfect.
13:54 I JUST THOUGHT ABOUT THOSE SHAPES AND THE WAYS TO MAKE THEM THIS WEEK!!!!!!!
I should really pay more attention to choosing colors. Is there a way to trick a computer screen into systematically recoloring things on the fly to make them distinguishable for color blind people :)
Swivel , just because I didn't originally catch the insight into the incircle... and the swivel has reflective properties that feels like decoding a secret message through a mirror - very cool. Colouring is ok. It's nice.
Thank you for such a nice and informative video. I liked The colour proof more than the swivel proof
I liked the color proof a lot more, it felt a lot more clear, straightforward and obvious to me!
Fair enough, it's a super slick one. Have you heard of the BOOK of proofs? Essentially what I asked you to vote on was which of the two proofs should go into the BOOK :)
en.wikipedia.org/wiki/Proofs_from_THE_BOOK
Oooh, that's awesome, thanks for the link and for the response! I learned something today :D
Personally I like to think of mathematics as inhabited by amazing creatures that move around interacting and spawning new creatures. So I was immediately attracted to the wiper approach.
That was a good one. Again I always watch this on a Sunday afternoon. Not a mathematician but my theory is that they are the same.
To assist your engagement; I liked the coloring better, the idea of the semi-sides each composing the half the chords is pleasing.
Thank you for assisting :)
The swiveling proof is very appeasing
*Proof of Circle* :
Given any two lines, they lie on a circle because the power of the intersection point is the same. This is the product of the lengths of the parts of any chord on which the point lines. If the lengths are a,b,c then the power of one of the points, evaluated through one line is a*(b+c), and evaluated through the other line is a*(c+b).
This makes the 4 points cyclic because given 3 of the points, we can define a circle, 2 of the points are on a single line and that gives the power of the point. We can then determine what point gives the correct power for the other line, and since the point at which the circle and the other line intersect gives that same power (and power is linear with point position along the line), this intersection point is the same as the 4-th point.
Now we find the center of the circle formed. Note that the two lines, lets name them ABC and ACB, are mirror reflections along the angle bisector since the lengths of the sections formed by the point are the same, a and (b+c). Thus the center lies along the angle bisector.
Note that the center also lies along the perpendicular bisector of the two chords ABC and ACB. The distance along the perpendicular bisector is a) identical, can be found bu evaluating position of perpendicular along the lines then noticing the equal angle, hence congruence.
Evaluating another pair of lines and checking the lengths, we see that the center point is the incircle center, and the distance along the perpendiculars is all the same (by congruence of triangles), and is the incircle radius, which are both known to be unique.
Also, the diameter of the circle has to be larger because two of the opposite points form the chords aka whiskers of the triangle, which is smaller than the circle's diameter.
Answer to the puzzle: The circle has a larger diameter.
Because the weird curve's diameter is exactly the same as the sum of the 3 sides of the triangle (can be seen because it meets the lines perpendicularly, by its construction).
The Conway circle is larger as it also passes through the same two points on any of those lines, but has a center which is not on the line.
Correct :)
Love it❤
I love both of the (perpendicular) bisection proofs SO MUCH.
Always cool T-shirts
Remember I was young and there was Conway's fan club on the next street: they listen punk rock, drink beer and has Conway's iris emblem on the jackets.
Seriously? Not a joke? :) Which Conway?
Since the wiper segments pivot from the same point when they wipe, the traced path from when they start to end wiping is on a circle with the pivot point as the midpoint of the circle. This means that the "weird curve" shape is made of overlapping circles!
Correct :)
5:41: The two yellow segments are congruent because they are formed by two tangent lines where they intersect the circle and each other. If you connect the two points where the tangents intersect the circle, the newly made angles are interior angles that intercept the same arc. If you use the formula { measure of angle = 1/2 measure of arc in degrees} you can set the angles equal. This makes the triangle isosceles, and the two yellow segments of the triangle congruent.
Diameter of the circle is larger. If you superimpose the circle over the shape, you can form a triangle of the shape's width and two radii of the circle. Due to triangle inequality, the two radii, and therefore the circle's diameter, must be longer.
Beautiful! The last animation makes me think of a rotary engine! Constant diameter shape seems like another way to design a rotary engine where the outer part could be simpler than the one actually used in rotary engines.
from triangle inequality the diameter of wiper curve is larger because if you take triangle with base of diameter of wiper and center of circle then 2r_circle < d_wiper
I personally find the proof with the two colors clearer than the proof wit the swivelling. Maybe it's just me, but I find saying that the sum of some numbers is equal to the sum of some other numbers more obvious than saying that swivelling some line segments around makes them coincide.
He made it symmetrical bc he is adding length of side b to where angle B intersects (etc)
Brilliant and fun video
The curve of constant width at the end was neat. Also, Conway's game of life is a simplification of a game invented by John Von Neumann. Von Neumann was studying the idea because he was thinking about self-replicating machines.
I kept seeing the straightedge and compass method for finding the incircle of a triangle, just the intersection of two angle bisectors in the triangle -- and I kept seeing angle bisectors of the vertical angles.
I liked Both Versions of the Proof, the coloured one is a Little Bit more intuitive. Thanks again for a Great Video ❤
I like the swivel proof and the color proof equally.
The Action Lab made a good video about shapes of constant width, he also constructs one.
Fun fact: if you make Conway's iris with a regular triangle, the radius of the outer circle is roughly 5.29 times (twice root 7 times) bigger than the radius of the inner circle (which was a fun algebraic challenge to compute).
The swivelling proof is better because it opens the idea to so many other uses in geometry.
Very neat video. That little twist at the end resembled somewhat the rotary wheel of a Wankel motor. Yes, I'm that old...
13:50 FINALLY I remembered from decades ago Martin Gardner's column on equal-width shapes, including this as a method of generating them.
Martin Gardner sure covered a LOT of ground in his writings :)
The proof is visible in the thumbnail picture. The whole line segments are of the same length which is the sum of the side lengths of the triangle. Looking at a pair of them, the shorter outer partitions are the opposite side length of the triangle (the partition has to be shorter because one side length of a triangle can never be longer than the other two), the longer ones have to be the sum of the other two side lengths. So its possible to construct a circle that goes through the four end points using the angle bisector and the normal line at the half point (of the whole length). We can do this with each pair and each pair shares a leg, so it becomes obvious each of the line segments is a secant of the same length of one similar outer circle. If we rotate them, they form the inner circle.
Edit: This took me a month, perhaps because it's so obvious.
The swivel proof is my favorite.
Woah! That was faster than I expected!
The coloring proof was the one I came up with. It was obvious that if the points were all on a circle then it had to be concentric with the inscribed circle of the triangle. Since the six pieces obtained when cutting the sides at the tangent points to the inscribed circle would be pairwise identical I could show that the cords were indeed bisected.
Nice!
My proof: label the 3 triangle lengths a,b,c, and their opposite angles A, B, C. connect 4 of the outer points, two from each of two pairs of the extensions (say two of extensions b, and two of extensions c, to form a quadrilateral. Draw from where the b's meet to the broad edge of the new quadrilateral a line with angle A. do the same from where the c's meet, again with angle A. Since each of those origination points have an angle C and A, (or B and A) and lie on a line, the interior angles are B and C respectively. There fore B, a, C must give you a congruent triangle to the original triangle. There fore you now have a couple more isoceles triangles. Now go to opposite corners. You can quickly find that, you'll get, for instance, angle 90 - C/2 opposite angles 90 -A/2 and 90 - B/2 on the other. These add up to 180, and so are supplementary, and so the new quadrilateral is an inscribed quadrilateral. Since there was nothing special about which of the three quadrilateral we drew, all three are inscribed quadrilaterals. You can draw more quadrilaterals in the same way, to find a bunch more of these half angles, and since 3 points define a circle, you can group them so that eventually you find that all 6 points are parts of a whole bunch of inscribed quadrilaterals that have to be on the same circle.
I like all your lines best! ❤🐓🦅🪿🦃🐣🦆🦜🐦🦢🦉🐔
This looks like an optical illusion, because the lines don't really appear to be of equal length!! Fascinating stuff!! Even though the professor maintains that they are all of equal length, or so I thought 💭 I heard him say!! Hmmm!!! Food for thought 💭
Nice video. My proof of the first part: it is enough to show that the quadrilateral formed by the a and c whiskers is cyclic. For this, we use the 3 isosceles triangles (two whisker triangles and one triangle with side length a+c). Then opposite angles are complementary.
I prefer the swivel proof because it's closer to the proof I came up with while I had the video paused. I'll admit I did use the thumbnail as a hint that the inscribed circle would be relevant, then proved each two adjacent points were the same distance from the incentre. Basically the same logic but without actually mentioning midpoints.
I realised after the video that I'd actually seen the windscreen wiper generation of a curve of constant width before - it's in Matt Parker's Things to Make and Do in the Fourth Dimension as a way to make curves of constant width with an even number of arcs, while regular Reuleaux polygons are always odd.
I like the swivel proof because it was nice, and the coloring proof seems like the more obvious sort of approach to take.
So did I, for the same reason.
The swivel proof did it for me. Simple and elegant. Plus I have a bit of a spiral fixation.😂
Definitely like the color one better. It keeps in mind the lengths of the original, arbitrary triangle.
Draw the lines between neighboring points. Notice that each one forms an isosceles triangle with (alternating) the inner and the outer angles of the original triangle. Choose any four consecutive points to form a quadrilateral. This is a cyclic quadrilateral as two opposing angles will sum to 360 degrees minus the sum of the angles of the original triangle, i.e. 180 degrees. Since I can repeat the argument for any group of four neighboring points, all these possible cyclic quadrilaterals must have the same circumscribing circle.
The colour proof is the one I would most likely find on my own. The swivel proof I think is more elegant and more appealing
I'm pausing to try to prove it, and I think I got it!
It's hard to explain without pictures, but the incircle meets each side of the triangle, splitting each into two segments. We can say red=a+b, blue=b+c, and green=c+a.
The line extended from the red side has a length of a+blue = a+b+c below its tangent point to the incircle. And it has a length of b+green = b+c+a above the tangent point. So it is tangent to the incircle at its midpoint.
The same goes for the other two extended lines. Since each line is the same length, and they're all balancing on the incircle at their midpoints, they are the same thing, just rotated around the incircle's center. Thus their endpoints all lie on a circle.
I came up with the color proof, but man that swivel proof was slick!
I think that this constant width shape isn't that surprising because it is a consequence of this windscreen wiper action, because we rotate the same length at different points so the diameter is always the same and equal to the length of this wiper
I like the swiveling proof better. Once you've observed that the two lengths from a corner to the points of intersection are equal, the rest follows with very little explanation. The colouring proof has its own appeal, but it feels a little clunkier.
7:38 the colored one is exactly the proof that i was able to get when paused at 2:12. I like this proof more as it shows, that for triangle with sides a, b and c, if we split a in two segments by the touching point of inscribed circle, then difference of these segments is equal to difference of remaining sides of the triangle.
Thanks alot. Very nice video.
Glad you liked it!