没错,单位向量的模长就是1,设a,b的夹角为α
cosα=a*b/(|a||b|)
a*b=2n^2+m*n-6m^2
n^2=|n|^2=1,m^2=|m|^2=1
所以a*b=-4+m*n
设m,n的夹角为β,β=60°则cosβ=m*n/(|m||n|)=cos60°=1/2
所以m*n=1/2
所以a*b=-4+1/2=-7/2
|a|^2=a^2=4m^2-4m*n+n^2=4+2+1=7
|a|=根号7
|b|^2=4n^2-12m*n+9m^2=9-6+4=7
|b|=根号7
所以cosα=-（7/2）/（根号7*根号7）=-1/2
α=arccos-1/2=120°