Chris Card
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### Answers (17)

 22 How Can One Prove $\cos(\pi/7) + \cos(3 \pi/7) + \cos(5 \pi/7) = 1/2$ 11 If the product of an invertible symmetric matrix and some other matrix is symmetric, is that other matrix also symmetric? 11 What does E mean in 9.0122222900391E-5? 10 Proofs that every natural number is a sum of four squares. 5 Area's of rectangle and circle

### Reputation (1,230)

 +30 What does E mean in 9.0122222900391E-5? +5 Simplification of expressions containing radicals +5 Proof that Pi is constant (the same for all circles), without using limits +10 How Can One Prove $\cos(\pi/7) + \cos(3 \pi/7) + \cos(5 \pi/7) = 1/2$

### Questions (2)

 37 Proof that Pi is constant (the same for all circles), without using limits 15 Simplification of expressions containing radicals

### Tags (31)

 22 popular-math 10 alternative-proof 22 trigonometry 8 combinatorics × 2 12 linear-algebra × 2 5 circle 11 notation 4 coloring 10 number-theory 4 differential-equations

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