tanx+tany=3(tanx)(tany)=-3tan(x+y)=(tanx+tany)/(1-tanxtany)=3/4[sin(x+y)]^2+[cos(x+y)]^2=1[sin(x+y)]^2=9/25,[cos(x+y)]^2=16/25,sin(x+y)cos(x+y)={[cos(x+y)]^2}tan(x+y)=12/25[sin(x+y)]^2-3sin(x+y)cos(x+...求公式，谢谢