# Math

In 1880, James Garfield contributed a new proof of geometry’s most famous right triangle theorem. Mathematical treasure: Garfield’s proof https://www.maa.org/press/periodicals/convergence/mathematical-treasure-james-a-garfields-proof-of-the-pythagorean-theorem Engraved portrait https://en.wikipedia.org/wiki/File:GARFIELD,_James_A-President_(BEP_engraved_portrait).jpg Gizmodo post https://gizmodo.com/james-garfield-was-the-only-u-s-president-to-prove-a-m-1037750658 Subscribe: https://www.youtube.com/user/MindYourDecisions?sub_confirmation=1 Send me suggestions by email (address at end of many videos). I may not reply but I do consider all ideas! If you purchase through these links,

This is a standard question to prepare for India’s famously challenging JEE Advanced exam. Here is one way to solve the question quickly. Thanks to Avinash for the suggestion! Special thanks this month to: Daniel Lewis, Robert Zarnke, Kyle, Mike Robertson. Thanks to all supporters on Patreon! http://www.patreon.com/mindyourdecisions Problem credit to David Altizio (section 3

Why five-sided figures pose a problem from Professor John Hunton – and a bit about the importance of Penrose Tiling. More links & stuff in full description below ↓↓↓ Professor Hunton works at the University of Leicester. NUMBERPHILE Website: http://www.numberphile.com/ Numberphile on Facebook: http://www.facebook.com/numberphile Numberphile tweets: https://twitter.com/numberphile Subscribe: http://bit.ly/Numberphile_Sub Videos by Brady Haran Patreon: http://www.patreon.com/numberphile

A tetrahedron and a pyramid have edges of equal length. If they are glued together on a triangular face with the vertices aligned, how many faces will the new shape have? This question is trickier than it looks! It stumped the test-makers. Playable demonstration: Steve Phelps has made a Geogebra worksheet that you can try

The Big Bang Theory TV is now part of math history: there’s a theorem named after Sheldon. Sources Siouxland News article https://siouxlandnews.com/news/local/morningside-college-professors-math-appearing-in-the-big-bang-theory 73 is the best number – Big Bang Theory clip Proof of the Sheldon conjecture https://math.dartmouth.edu/~carlp/sheldon02132019.pdf 73 is the best number (other properties) Subscribe: https://www.youtube.com/user/MindYourDecisions?sub_confirmation=1 Playlist to watch all videos on MindYourDecisions: https://www.youtube.com/playlist?list=UUHnj59g7jezwTy5GeL8EA_g

Can you solve this problem that stumped many advanced math students? References https://io9.gizmodo.com/ready-this-simple-puzzle-once-stumped-96-of-americas-1698814691 https://www.theguardian.com/science/2017/jun/05/did-you-solve-it-are-you-in-the-smartest-10-per-cent https://www.mapleprimes.com/posts/206713-Puzzle-Or-A-Simple-Exercise http://puzzles.nigelcoldwell.co.uk/seventytwo.htm https://timss.bc.edu/timss1995i/TIMSSPDF/C_full.pdf Subscribe: https://www.youtube.com/user/MindYourDecisions?sub_confirmation=1 Send me suggestions by email (address in video). I consider all ideas though can’t always reply! Like many YouTubers I use popular software to prepare my videos. You can search for animation software tutorials on YouTube

There are 25 mechanical horses and a single racetrack. Each horse completes the track in a pre-programmed time, and the horses all have different finishing times, unknown to you. You can race 5 horses at a time. After a race is over, you get a printout with the order the horses finished, but not the

Colin Rizzio became famous after finding a mistake on the S.A.T. Sources https://www.deseret.com/1997/2/7/19293720/math-question-on-the-sat-didn-t-sit-well https://people.com/archive/flunking-the-tester-vol-47-no-7/ https://www.chicagotribune.com/news/ct-xpm-1997-02-07-9702070268-story.html https://www.dartmouth.edu/~chance/chance_news/recent_news/chance_news_6.03.html Subscribe: https://www.youtube.com/user/MindYourDecisions?sub_confirmation=1 Send me suggestions by email (address in video). I consider all ideas though can’t always reply! Like many YouTubers I use popular software to prepare my videos. You can search for animation software tutorials on YouTube to

Thanks to Dhrubajyoti for the suggestion! A similar problem was asked to class 10 students in India (Q33 from SMO junior 2016). I admit that this one stumped me. Can you figure it out? Solution presented: post by Thomas Hill on Quora https://www.quora.com/In-a-triangle-ABC-AB-AC-angle-BAC-100-circ-Let-D-be-a-point-such-that-B-is-an-interior-point-of-AD-and-AD-BC-How-can-you-find-angle-BDC Other solutions http://jwilson.coe.uga.edu/EMT668/EMT668.Folders.F97/Rapley/EMT725/100deg/100deg.html http://jwilson.coe.uga.edu/EMT668/EMAT6680.F99/Wise/essay1/essay1.htm https://gogeometry.blogspot.com/2011/11/problem-685-isosceles-triangle-100-40.html pic.twitter.com/NNSkNX2I5w — José Luis da Vila (@jldavilaa01)

Large factorials and the use of Stirling’s Approximation. Featuring Professor Ken McLaughlin. More links & stuff in full description below ↓↓↓ Professor McLaughlin is based at Colorado State University: https://www.math.colostate.edu/~kenmcl/ We filmed this during his time at the Mathematical Sciences Research Institute. 69 Factorial: https://www.youtube.com/watch?v=kw6l_uTakRA Numberphile is supported by the Mathematical Sciences Research Institute (MSRI):

Thanks to Ryan from India for the suggestion! Two candles of equal height but different thickness are lit. The first burns off in 8 hours and the second in 10 hours. When will the first candle be half the height of the second? Subscribe: https://www.youtube.com/user/MindYourDecisions?sub_confirmation=1 Send me suggestions by email (address at end of many

Can you solve this neat geometry puzzle? Thanks to Joe M. for the suggestion! Sources https://brilliant.org/practice/circle-properties-level-2-3-challenges/?problem=day-5-five-golden-circles&subtopic=circles&chapter=circle-properties How I’m Learning to Step into Math Problems Circle puzzle Subscribe: https://www.youtube.com/user/MindYourDecisions?sub_confirmation=1 Send me suggestions by email (address in video). I consider all ideas though can’t always reply! Like many YouTubers I use popular software to prepare my videos.

A compilation of Numberphile sound checks – Brady typically asks “what did you have for breakfast” so he can monitor audio levels. More links & stuff in full description below ↓↓↓ Previous sound checks with scientists here: http://youtu.be/4anqDfgQneA Support us on Patreon: http://www.patreon.com/numberphile NUMBERPHILE Website: http://www.numberphile.com/ Numberphile on Facebook: http://www.facebook.com/numberphile Numberphile tweets: https://twitter.com/numberphile Subscribe: http://bit.ly/Numberphile_Sub

17 is the minimum number of clues required to give a unique sudoku solution – but how did mathematicians prove this? Featuring James Grime. More links & stuff in full description below ↓↓↓ Dr James Grime discusses a recent paper which cracked the problem. The paper being discussed by McGuire and others is at http://arxiv.org/abs/1201.0749

A 6’28” version of our “Pi with Pies” calculation. More links & stuff in full description below ↓↓↓ Videos features Matt Parker who tweets at https://twitter.com/standupmaths The Pi-inspired music in the video explained: http://periodicvideos.blogspot.co.uk/2013/03/pimusic.html Our full collection of Pi videos at http://bit.ly/W4oDN1 While the pies are usually delicious, this batch was not for consumption (even

Thanks to Amit from India for the suggestion! A circle is tangent on the exterior of a side of a triangle and tangent to the lines through the other two sides. The tangent side length 7, and the other side lengths are 8 and 9. What is the radius of the circle? 0:00 Problem 1:22

Our Pi Playlist (more videos): http://bit.ly/PiPlaylist How accurately can we calculate Pi using hundreds of REAL pies? More links & stuff in full description below ↓↓↓ Extended version of this video (director’s slice) at http://www.youtube.com/watch?v=x4kyFKyCMv0 This video features Matt Parker: https://twitter.com/standupmaths Matt believes this is the world’s most accurate pie-based Pi calculation. More Pi videos:

Thanks to Victor for the suggestion! A version of this problem appeared on a Kangaroo math competition for 12-13 year olds. Can you figure it out? Special thanks this month to: Michael Anvari, Daniel Lewis, Robert Zarnke, Kyle, Mike Robertson. Thanks to all supporters on Patreon! http://www.patreon.com/mindyourdecisions Original Math Kangaroo question on YouTube Subscribe: https://www.youtube.com/user/MindYourDecisions?sub_confirmation=1

Matt Parker explores the work of William Shanks – and boots up the ShanksBot. More links & stuff in full description below ↓↓↓ Matt Parker’s 2022 Pi Day Video: https://youtu.be/dtiLxLrzjOQ Discussing William Shanks on Objectivity: https://youtu.be/7yTXMeiVBCc Prime Number playlist: https://bit.ly/PrimePlaylist Pi playlist: http://bit.ly/PiPlaylist Matt Parker website: https://standupmaths.com Numberphile is supported by the Mathematical Sciences Research

I didn’t solve this problem myself, but I felt better when I learned WolframAlpha couldn’t solve it either! But there is a way to solve it using careful mathematical reasoning. Thanks to Luka Khizambareli from Georgia for suggesting this and sending its solution! (*And you have to be really careful–I thank Aniket Gupta and Ryan

Cliff Stoll is passionate about Klein Bottles. More links & stuff in full description below ↓↓↓ Don’t miss the video about how he uses a robot to store 1,000 bottles UNDER his house… https://youtu.be/-k3mVnRlQLU More videos on Klein Bottles: http://bit.ly/KleinBottles ACME Klein Bottles: http://bit.ly/ACME_Klein Support us on Patreon: http://www.patreon.com/numberphile NUMBERPHILE Website: http://www.numberphile.com/ Numberphile on Facebook:

People often email me that my videos helped them get a tech or finance job, which is really awesome, and I want to do more to help! So in this video I’m sharing a problem based on an interview question asked at Facebook. Source https://www.glassdoor.com/Interview/You-re-about-to-get-on-a-plane-to-Seattle-You-want-to-know-if-you-should-bring-an-umbrella-You-call-3-random-friends-of-y-QTN_519262.htm (Note: I re-uploaded this video with corrections. The original calculations

Another pass at the Monty Hall Problem – see the last video and a new “express explanation” at: http://bit.ly/MontyHallProb More links & stuff in full description below ↓↓↓ Support us on Patreon: http://www.patreon.com/numberphile NUMBERPHILE Website: http://www.numberphile.com/ Numberphile on Facebook: http://www.facebook.com/numberphile Numberphile tweets: https://twitter.com/numberphile Subscribe: http://bit.ly/Numberphile_Sub Numberphile is supported by the Mathematical Sciences Research Institute (MSRI):

Thanks to Devan from Lilin Bangsa Intercultural School for suggesting this problem! What is the radius of the small circle in between the blue circle (radius 4) and green circle (radius 2)? This was a challenge problem for students aged 14 to 15. Thanks to all patrons! Special thanks to: Shrihari Puranik Kyle Professor X

The harmonic series and the elusive Euler–Mascheroni constant. More links & stuff in full description below ↓↓↓ Featuring Dr Tony Padilla. Audible: http://www.audible.com/numberphile Extra footage: https://youtu.be/eRGN8ThZfhU Videos about -1/12: http://bit.ly/minus_twelfth Tony at the LHC: https://youtu.be/sVYUqMRolaA (via Sixty Symbols, our physics channel) Support us on Patreon: http://www.patreon.com/numberphile NUMBERPHILE Website: http://www.numberphile.com/ Numberphile on Facebook: http://www.facebook.com/numberphile Numberphile tweets:

A semicircle contains an inscribed semicircle dividing its diameter into two lengths a and b. Can you find the formula for the inscribed semicircle’s diameter in terms of the lengths a and b? What is the locus of the center of the inscribed semicircle? Thanks to Nick from Greece for the suggestion! Special thanks this

Tadashi Tokieda is back. This time talking about stability, instability and train wheels. More Tadashi videos: http://bit.ly/tadashi_vids Numberphile is supported by the Mathematical Sciences Research Institute (MSRI): http://bit.ly/MSRINumberphile We are also supported by Science Sandbox, a Simons Foundation initiative dedicated to engaging everyone with the process of science. NUMBERPHILE Website: http://www.numberphile.com/ Numberphile on Facebook: http://www.facebook.com/numberphile

Thanks to Reio in Romania for emailing me this fun problem! What is the area? This puzzle was shared with the tagline “you should be able to solve this.” Solution to 5th grade Chinese challenge problem My blog post for this video https://wp.me/p6aMk-82G Reference Math problem from anime Steins;Gate https://math.stackexchange.com/questions/798367/area-of-a-part-of-a-square Subscribe: https://www.youtube.com/user/MindYourDecisions?sub_confirmation=1 Send me suggestions

Photomath can’t solve this, and neither can Mathway, Symbolab, or Desmos. Can you? I explain how you can. A similar problem was given to high school students in Massachusetts, and you really have to know what you’re doing to figure it out. Thanks to all patrons! Special thanks this month to: Shrihari Puranik Richard Ohnemus

Solve this to get into Oxford! 2021 Oxford MAT Questions https://www.maths.ox.ac.uk/system/files/attachments/test21.pdf 2021 Oxford MAT Solutions https://www.maths.ox.ac.uk/system/files/attachments/websolutions21.pdf Subscribe: https://www.youtube.com/user/MindYourDecisions?sub_confirmation=1 Send me suggestions by email (address at end of many videos). I may not reply but I do consider all ideas! If you buy from the links below I may receive a commission for sales. (As an

This is a fun little geometry puzzle! adapted from a puzzle by @Cshearer41 What’s the area of the toppled square? pic.twitter.com/77yHacED5Y — Catriona Agg (@Cshearer41) August 7, 2018 solution 1- area multiplication right squares: 3*4 = 12 and 3*9 = 27, that means *2 and *3 for the sides. 2- roots for sides: √3, 2√3

Hannah Fry on parallels between the game “rock paper scissors” and lizards in nature. PART ONE: http://youtu.be/rudzYPHuewc More links & stuff in full description below ↓↓↓ EXTRA FOOTAGE: http://youtu.be/ygHwBxWyI6E Lizards by Pete McPartlan Support us on Patreon: http://www.patreon.com/numberphile NUMBERPHILE Website: http://www.numberphile.com/ Numberphile on Facebook: http://www.facebook.com/numberphile Numberphile tweets: https://twitter.com/numberphile Subscribe: http://bit.ly/Numberphile_Sub Numberphile is supported by the

Alex’s book: http://amzn.to/1l0yX46 The Curta is a pocket-sized, mechanical, digital calculator!!! It was invented by Curt Herzstark. More links & stuff in full description below ↓↓↓ Shown here by Alex Bellos, author of Alex’s Adventures in Numberland. More about our contributors, including Alex, at http://www.numberphile.com/team/index.html NUMBERPHILE Website: http://www.numberphile.com/ Numberphile on Facebook: http://www.facebook.com/numberphile Numberphile tweets: https://twitter.com/numberphile

Thanks to Lucas for suggesting this problem! It comes from a Belgian Olympiad problem for 16 to 18 year old students. Subscribe: https://www.youtube.com/user/MindYourDecisions?sub_confirmation=1 Send me suggestions by email (address in video). I consider all ideas though can’t always reply! Like many YouTubers I use popular software to prepare my videos. You can search for animation

Featuring Professor Hannah Fry – more details on her work below. Check out Brilliant (get 20% off their premium service): https://brilliant.org/numberphile (sponsor) More links & stuff in full description below ↓↓↓ Hannah Fry: https://hannahfry.co.uk Hannah’s books: https://amzn.to/3ArNEaA Her latest is Rutherford and Fry’s Complete Guide to Absolutely Everything: https://amzn.to/3tSvYDN More Hannah Fry on Numberphile: http://bit.ly/hannah_vids

Squarespace: http://www.squarespace.com/numberphile This video features Hannah Fry – https://twitter.com/fryrsquared More links & stuff in full description below ↓↓↓ More on this topic (and lizards): http://youtu.be/Z8lv2vy5vco And even more on this topic: http://youtu.be/ygHwBxWyI6E The paper: http://bit.ly/RPSpaper Reddit for this video: http://redd.it/2tq25k Art and animation by Pete McPartlan Support us on Patreon: http://www.patreon.com/numberphile NUMBERPHILE Website: http://www.numberphile.com/ Numberphile

These are the answers to the previous video with 3 matchstick puzzlers. If you like my videos, you can support me at Patreon: http://www.patreon.com/mindyourdecisions Connect on social media. I update each site when I have a new video or blog post, so you can follow me on whichever method is most convenient for you. My

What is the radius of each sphere? Reference http://puzzling.stackexchange.com/questions/41952/nine-identical-spheres-fit-exactly-into-a-cube Subscribe: https://www.youtube.com/user/MindYourDecisions?sub_confirmation=1 Send me suggestions by email (address in video). I consider all ideas though can’t always reply! Like many YouTubers I use popular software to prepare my videos. You can search for animation software tutorials on YouTube to learn how to make videos. Be prepared–animation

Thanks to Umesh for the suggestion! If the large circle has a radius equal to 1, what is the radius of each small circle? Subscribe: https://www.youtube.com/user/MindYourDecisions?sub_confirmation=1 Send me suggestions by email (address at end of many videos). I may not reply but I do consider all ideas! If you buy from the links below I

Happy Pi Day! At 3/14/15 at 9:26 the time will be accurate to pi for 7 decimal places. What’s your favorite fact about pi? Links to proofs and more below in the description. 1. The definition 00:10 Related: the area of circle intuitive explanation 2. Pi is irrational 00:47 Proof (video) Proof (blog post) Proving

I bet this one will give many adults a good challenge too. Thanks to Pett for the suggestion! Special thanks this month to: Kyle, Mike Robertson, Michael Anvari, Daniel Lewis, Robert Zarnke. Thanks to all supporters on Patreon! http://www.patreon.com/mindyourdecisions Singapore International Math Olympiad Challenge (SIMOC) 2015 – Primary 5 Paper, problem 24. Sample Papers Subscribe:

We’re exploring the world of Chicken Nuggets and Frobenius numbers. 43 has long been a special number in the world of McNugget mathematics, but 11 is also important! More links & stuff in full description below ↓↓↓ This video features Dr James Grime. NUMBERPHILE Website: http://www.numberphile.com/ Numberphile on Facebook: http://www.facebook.com/numberphile Numberphile tweets: https://twitter.com/numberphile Subscribe: http://bit.ly/Numberphile_Sub

How do you measure the length of a spiral on a cylinder? There is a neat trick! If you like my videos, you can support me at Patreon: http://www.patreon.com/mindyourdecisions Connect on social media. I update each site when I have a new video or blog post, so you can follow me on whichever method is

Featuring Grant Sanderson, creator of 3blue1brown. Extra footage from this interview: https://youtu.be/pJyKM-7IgAU 3blue1brown video on the shadow a cube: https://youtu.be/ltLUadnCyi0 More links & stuff in full description below ↓↓↓ 3blue1brown: https://www.youtube.com/channel/UCYO_jab_esuFRV4b17AJtAw Grant on The Numberphile Podcast: https://youtu.be/A0RH93XvSyU Numberphile is supported by the Mathematical Sciences Research Institute (MSRI): http://bit.ly/MSRINumberphile We are also supported by Science Sandbox,

Catch a more in-depth interview with Ben Sparks on our Numberphile Podcast: https://youtu.be/-tGni9ObJWk Check out Brilliant (and get 20% off) by clicking https://brilliant.org/numberphile More links & stuff in full description below ↓↓↓ Golden seeds limited edition T-Shirt: https://teespring.com/NP-Seeds More Golden Ratio stuff: http://bit.ly/Golden_Ratio More Ben Sparks Numberphile videos: http://bit.ly/Sparks_Playlist Ben’s Twitter: https://twitter.com/SparksMaths Ben’s website: www.bensparks.co.uk

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