已知sin(π-α)cos(-α+3π/2)/cos(-π-α),且α为第三象限角1.化简f(α)2.若cos（α+π/2）=1/5,求f（α）的值

题目

已知sin(π-α)cos(-α+3π/2)/cos(-π-α),且α为第三象限角1.化简f(α)2.若cos（α+π/2）=1/5,求f（α）的值

答案

f(α) = sin(π-α)cos(-α+3π/2)/cos(-π-α)= sinα cos(-α+2π-π/2)/cos(π+α)= sinα cos(α+π/2)/(-cosα)= sinα (-sinα)/(-cosα)= sin^2α/cosαα为第三象限角cos（α+π/2）=1/5-sinα = 1/5sinα =...