a.b=(3,-2).(4,1)=12-2=10
|a+b|=|(3,-2)+(4,1)|=|(7,-1)|=√[7^2+(-1)^2]=√50=5√2
设a与b的夹角θ
cosθ=a.b/(|a|*|b|)=10/[√(3^2+(-2)^2)*√(4^2+1^2)]
=10/√221