Question

已知绝对值a=2,绝对值b=4,a与b的夹角为3分之π.求：以a,b为邻边的平行四边形对角线的长.

Answer 1

c^2=a^2+b^2-2abcosπ/3

or

c^2=a^2+b^2-2abcos2π/3