向量问题,求解答!

题目

向量问题,求解答!
已知|a|=1,a*b=1/2,（a-b）*（a+b）=1/2,a+b与a-b的夹角为θ,则cosθ的值为

答案

(a+b).(a-b) =1/2
|a|^2-|b|^2=1/2
|b|^2 = 1/2
|a+b|^2= |a|^2+|b|^2+2a.b
= 1+ 1/2 + 1
= 5/2
|a+b| =√(5/2)
|a-b|^2= |a|^2+|b|^2-2a.b
= 1+ 1/2 - 1
= 1/2
|a+b| =1/√2
(a+b).(a-b) = |a+b||a-b|cosθ
1/2 = (√5/2) cosθ
cosθ = 1/√5