f(α)=(sinα*cosα*-tanα)/tan(π+α)*sin(π-α)=(-sinα*cosα*tanα)/tanα*sinα=-cosαcos(α-3π/2)=cos(α-3π/2+2π)=cos(α+π/2)=-sinα=1/5∵α是第三象限角,∴cosα＜0∴cosα=-2√6/5∴f(α)=-(-2√6...∴cosα=-2√6/5∴f(α)=-(-2√6/5)=2√6/5为什么等于这个啊已经化简f(α)得到 f(α)=-cosα