tan(π+α)=-1/2求sin(α-7π)·cos(α+5π)
问题描述：
tan(π+α)=-1/2求sin(α-7π)·cos(α+5π)
答
tan(π+α)=tanα=-1/2 sin(α-7π)·cos(α+5π) =-sinα*（-cosα） =sinα*cosα/[(sinα)^2+(cosα)^2] =tanα/[(tanα)^2+1] =-1/2/[(-1/2)^2+1] =-2/5