(2a-3b)*(2a+b)=61
4a²-4a.b-3b²=61
64-4a.b-27=61
4a.b=-24
(1)a.b=-6
(2) 设夹角为θ
cosθ=a.b/(|a|*|b|0=-6/(4*3)=-1/2
所以夹角是120°
(3) (a+b)²=a²+b²+2a.b=16+9-12=13
a+b的值=√13