Question

急啊已知|a|=3,|b|=4,=60°,求(a+2b)·(a-3b)

Answer 1

（a+2b）.(a-3b)

=/a/^2-6/b/^2-a.b

=/a/^2-6/b/^2-/a/./b/cos60°

∵/a/=3,/b/=4

∴上式=3^2-6*(4)^2-3*4cos60°=9-96-6=-93