设向量a+b=(16,-8)与向量a-b=(-8,2)求向量a与b夹角θ 的余弦

a+b=(16,-8) (1)
a-b=(-8,2) (2)
(1)+(2)
2a=(8,-6)
a=(4,-3)
(1)-(2)
b = (12,-5)
|a|=5
|b|=13
|a||b|cosθ =a.b
65cosθ = (4,-3).(12,-5)
=48+15
cosθ = 63/65