1^2+2^2+3^2+4^2+...+n^2 = n*(n +1)*(2*n +1)/6;

http://wenku.baidu.com/view/b481511fb7360b4c2e3f6470.html

余弦定理:

cosA=(b²+c²-a²)/(2bc)
cosB=(a²+c²-b²)/(2ac)
cosC=(a²+b²-c²)/(2ab)