|a|=3,|b|=4,向量a与向量b的夹角为3π/4
a*b=|a||b|cos3π/4=3*4*(-√2/2)=-6√2
a^2=|a|^2=9,b^2=|b|^2=16
(3a-b)*(a+2b)= 3a^2-2b^2+5a*b
=3*9-2*16-30√2=-5-30√2
|a|=3,|b|=4,向量a与向量b的夹角为3π/4
a*b=|a||b|cos3π/4=3*4*(-√2/2)=-6√2
a^2=|a|^2=9,b^2=|b|^2=16
(3a-b)*(a+2b)= 3a^2-2b^2+5a*b
=3*9-2*16-30√2=-5-30√2