1 (a+b)*(a-2b) =a^2-a*b-2b^2 =9-3*4 2-32 =-29 |a-b|^2=a^2-2a*b+b^2 =9-2*3*4 2+16 =13, |a-b|=√13 2 向量AB*AC=6*7*cos150° =-21√3 。
