1.（a+2b)乘（a-3b)=-72,所以|a|^2-ab-6|b|^2=-72
所以|a|^2-|a||b|cos60-24=0.|a|^2-2|a|-24=0,|a|=6或-4(-4舍去）.
2.because ab=0,so 3*2+m=0,m=-6
3.because ...,so (a+2b)*(3a-b)=0,3a^2+5ab-2b^2=0,12+5*2*4*cosm-2*4^2=0,m=60度
4.because ...,so ab-a^2=2,1*6*cosm-1=2,m still equal to 60 grad.
给我重赏呀!