问题1.万能公式
sinx=2tan(x/2)/(1+tan(x/2)*tan(x/2))=2t/(1+t^2);
问题2.(sinx+siny)^2=1-------------------①
（cosx+cosy)^2=t-------------------②
①+②
2+2cos(x-y)=t+1-------------------③
t>=0------------------------------④
-1=