Guy

 136 Visually stunning math concepts which are easy to explain 29 Can a piece of A4 paper be folded so that it's thick enough to reach the moon? 17 Logic behind dividing negative numbers 15 A problem on divisibility theorem 14 Find the integral part of $\sum_{i=2}^{10000}\frac1{\sqrt{i}}$

### Reputation (8,345)

 +10 How to prove this inequality $x + \frac{1}{x} \geq 2$ +10 Finding the “triangular root” of a number. +10 If p $\equiv$ 3 (mod 4) with p prime, prove -1 is a non-quadratic residue modulo p. +10 Logic behind dividing negative numbers

### Questions (41)

 49 What exactly are fractals 13 Prove $1^2+2^2+\cdots+n^2 = {n+1\choose2}+2{n+1\choose3}$ 10 Prove $\sum_{n=1}^\infty(e-\sum_{k=0}^n\frac1{k!})=1$ 9 Why are mathematical results discovered by multiple people independently? 8 Historic proof of the area of a circle

### Tags (138)

 75 algebra-precalculus × 32 38 sequences-and-series × 17 70 elementary-number-theory × 21 35 integration × 23 68 calculus × 40 29 recreational-mathematics × 6 58 summation × 21 24 approximation × 6 55 arithmetic × 4 23 number-theory × 15

### Bookmarks (18)

 1037 How long will it take Marie to saw another board into 3 pieces? 924 Can I use my powers for good? [closed] 800 The staircase paradox, or why $\pi\ne4$ 180 What Does it Really Mean to Have Different Kinds of Infinities? 114 Find the average of $\sin^{100} (x)$ in 5 minutes?