$1+2^2+3^2+\cdots+n^2 = \frac{1}{3}n(n+1)\left(n+\frac{1}{2}\right)=\frac{n(n+1)(2n+1)}{6}$ This originally came in Mathematics Magazine (1984 edition). The author of this ingenious proof is Prof. Siu Man Keung. This link provides further information regarding the article.
Viviani's Theorem states that for any point $\mathcal{P}$ inside an equilateral triangle $\mathcal{ABC}$ the sum of the lengths of perpendiculars drawn from P on the sides is equal to the length of the altitude. Draw equilateral triangles from that point onto the sides of the triangle such that these 3 perpendiculars are the altitudes of the equialteral triangles.Now rotate the triangles as shown.Now it can be easily seen that the sum of the 3 perpendiculars=perpendicular of the $\Delta ABC$
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