tan(A+B)=2tanAsin(A+B)*cosA=2sinA*cos(A+B) ①sin(A+B)*cosA-sinA*cos(A+B)=sinA*cos(A+B)sinB=sinA*cos(A+B)sin(2A+B)=sin(A+B)*cosA+sinA*cos(A+B)=3sinA*cos(A+B)（因为①）3sinB=sin(2A+B)