|a+b|=根号13,|a+b|^2=a^2+2a·b+b^2=139+2a·b+b^2=13a·b=|a||b|cos120=3*|b|*(-1/2)即2*(-3/2 |b|)+b^2=4b^2-3|b|-4=0(|b|-4)(|b|+1)=0所以|b|=4
