Dana Ernst
September 18, 2015
450

# Proofs Without Words

In this FAMUS talk, we'll explore several cool mathematical theorems from a visual perspective.

This talk was given at the Northern Arizona University Friday Afternoon Mathematics Undergraduate Seminar (FAMUS) on Friday, September 15, 2015.

## Dana Ernst

September 18, 2015

## Transcript

1. ### Proofs Without Words Dana C. Ernst Northern Arizona University Mathematics

& Statistics Department http://dcernst.github.io Friday Afternoon Mathematics Undergraduate Seminar September 18, 2015 D.C. Ernst Proofs Without Words 1 / 17
2. ### Warning! Pictures can be misleading! Theorem? Hmmm, it looks like

32.5 = 31.5. D.C. Ernst Proofs Without Words 2 / 17
3. ### Play Time Let’s play a game. • I’ll show you

a picture, • You see if you can ﬁgure out what mathematical fact it describes or proofs. D.C. Ernst Proofs Without Words 3 / 17

/ 17
5. ### Theorem For all n ∈ N, 1 + 2 +

· · · + n = n(n + 1) 2 . Note The numbers Tn := 1 + 2 + · · · + n are called triangular numbers. D.C. Ernst Proofs Without Words 5 / 17
6. ### Theorem For all n ∈ N, 1 + 2 +

· · · + n = C(n + 1, 2) := (n + 1)! 2!(n − 1)! . Corollary For all n ∈ N, C(n + 1, 2) = n(n + 1) 2 . D.C. Ernst Proofs Without Words 6 / 17
7. ### Theorem For all n ∈ N, 1 + 3 +

5 + · · · + (2n − 1) = n2. D.C. Ernst Proofs Without Words 7 / 17
8. ### This the same as the previous theorem, but with a

diﬀerent visual proof. Theorem For all n ∈ N, 1 + 3 + 5 + · · · + (2n − 1) = n2. D.C. Ernst Proofs Without Words 8 / 17
9. ### The nth pentagonal number is deﬁned to be Pn :=

3n2 − n 2 . Theorem Pn = 3Tn−1 + n. D.C. Ernst Proofs Without Words 9 / 17
10. ### Theorem (Nicomachus’ Theorem) For all n ∈ N, 13 +

23 + · · · + n3 = (1 + 2 + · · · + n)2. Corollary For all n ∈ N, 13 + 23 + · · · + n3 = ( n(n + 1) 2 ) 2 . D.C. Ernst Proofs Without Words 10 / 17
11. ### Theorem We have the following summation formula: ∞ ∑ k=1

( 1 2 ) k = 1. D.C. Ernst Proofs Without Words 11 / 17
12. ### Hint Focus on a single color. Theorem We have the

following summation formula: ∞ ∑ k=1 ( 1 4 ) k = 1 3 . D.C. Ernst Proofs Without Words 12 / 17
13. ### Theorem We have the following summation formula: ∞ ∑ k=1

( 1 3 ) k = 1 2 . D.C. Ernst Proofs Without Words 13 / 17
14. ### Theorem A circle of radius r has area πr2. D.C.

Ernst Proofs Without Words 14 / 17
15. ### Theorem The alternating sum of the ﬁrst n odd natural

numbers is n. In other words, for all n ∈ N, n ∑ k=1 (−1)n−k(2k − 1) = n. D.C. Ernst Proofs Without Words 15 / 17
16. ### Theorem We have the following fact concerning integrals: ∫ π/2

0 sin2(x) dx = π 4 = ∫ π/2 0 cos2(x) dx. D.C. Ernst Proofs Without Words 16 / 17
17. ### Sources MathOverﬂow: mathoverflow.net/questions/8846/proofs-without-words Art of Problem Solving: artofproblemsolving.com/Wiki/index.php/Proofs_ without_words Wikipedia:

en.wikipedia.org/wiki/Squared_triangular_number Strogatz, NY Times: opinionator.blogs.nytimes.com/2010/04/04/ take-it-to-the-limit/ D.C. Ernst Proofs Without Words 17 / 17