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
tan(π+α)=-1/2求sin(α-7π)·cos(α+5π)
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