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