tan(A+B)=sin(A+B)/cos(A+B)=(sinAcosB+sinBcosA)/(cosAcosB-sinAsinB)分子,分母同时除以cosAcosB得:=(sinA/cosA+sinB/cosB)/(1-sinAsinB/cosAcosB)=(tanA+tanB)/(1-tanAtanB)
