A parallelogram has sides measuring 7 and 9. Its shorter diagonal has a length of 8. Find the measure of the longer diagonal.
using law of cosines 8^2 = 7^2+9^2 - 2*7*9 cos A 64 = 49 + 81 - 126 cos A cos A = 66/126 x^2 = 130 - 126 cos(180-A) but cos(180-A) = -cos A so x^2 = 130 + 126(66/126) x^2 = 196 x = 14
