Question

已知|a|=6,|b|=4,a与b的夹角为60°，则(a+2b)·(a-3b)等于(    )

A.72                  B.-72                     C.36                   D.-36

Answer 1

解析:a·b=|a||b|cos60°=6×4×=12,

(a+2b)·(a-3b)=a²-a·b-6b²

=|a|²-a·b-6|b|²

=36-12-6×16=-72.

答案:B