b=?错了错了，b=-3m+2na=2m+n，|a|^2 =(2m+n).(2m+n) = 4|m|^2 +n^2 + 4m.n = 4+1 + 2= 7所以|a| = √7 b= -3m+2n，|b|^2 = 9 + 4 - 6 = 7，|b| = √7设 a,b 的夹角 =x，a.b=(2m+n).(-3m+2n)= -6|m|^2+2|n|^2 +m.n= -6 +2 +1×1×cos60°= -7/2 = |a||b|cosx7cosx = -7/2cosx = -1/2x = 120°