tg(x+y)=(tgx+tgy)/(1-tgx*tgy)
又tanx,tany是方程x^2+6x+7=0的两个根,所以tgx+tgy=-6,tgx*tgy=7
所以tg(x+y)=1,所以sin(x+y)=cos(x+y).
已知tanx,tany是方程x^2+6x+7=0的两个根,求证sin(x+y)=cos(x+y).
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