已知IaI=1,IbI=根号3,a+b=(根号3,1）,求Ia-bI ,向量a+b与向量a-b的夹角

let (a+b),(a-b)的夹角=x|a+b|^2= 4 = |a|^2+|b|^2+2a。b4 = 1+3+2a。ba。b =0 |a-b| = |a+b| =2(a-b)。(a+b) = |a+b||a-b|cosx|a|^2-|b|^2= |a+b||a-b|cosx1-3= 4cosxcosx = -1/2x= 120度