Question

已知向量ab满足[a+2b][a-b]=-6 ,|a|=1 b|=2 则ab夹角为

Answer 1

(a+2b)*(a-b)=-6

|a|^2-2|b|^2+ab=-6

ab=-6-1+2*2^2=1=|a||b|cos=1*2*cos

cos=1/2

=π/3