5sinB=sin(2A+B)
5sin[(A+B)-A]=sin[(A+B)+A]
5sin(A+B)cosA-5cos(A+B)sinA=sin(A+B)cosA+cos(A+B)sinA
4sin(A+B)cosA=6cos(A+B)sinA
所以,
4tan(A+B)=6tanA
所以,
tanA = (2/3)tan(A+B)
=3/2