由tan(A+B)=3tanA可以得到：sin(A+B)cosA=3sinAcos(A+B)展开,得(sinAcosB+cosAsinB)*cosA=3sinA*(cosAcosB-sinAsinB)去括号,移项,得：(cosA)^2*(sinB)=2sinAcosAcosB-3(sinA)^2*sinB再移项,化简得到：sinB*(1+2(sinA...