设m,n是两个单位向量,其夹角是120°,求向量a=2m+n,与b=3m +2n的夹角的余弦值.m^2=∣m∣^2=1n^2=∣n∣^2=1mn=∣m∣*∣n∣cos120°=1*1*(-0.5)= -0.5ab=(2m+n)( 3m +2n)= 6m^2+7mn+2n^2=6*1+7*(-0.5)+2*1=4.5a^2=(2m+n...