先化简
cos(π/2+α) = cos(π/2 -(-α)) = sin(-α) =-sinα
sin(α-π/2) = sin(-(π/2-α)) = -sin(π/2-α)= - cosα
sin（π-α）= sinα
cos（π+α）= -cosα
cos（5π/2 -α）= cos（4π/2 + π/2 -α）= cos(π/2 -α) = sinα
sin（7π/2 -α）= sin（8π/2 -π/2-α） = sin(-π/2-α)= -sin(π/2-(-α) )= -cos(-α) = -cosa
由已知-sinα= - cosα
所以sinα=cosα
设sinα=cosα = p
sin（π-α）-cos（π+α）/cos（5π/2 -α）+2sin（7π/2 -α）
= sinα - (-cosα) / sinα + 2(-cosα)
= p +p / p -2p
= 2p/-p
= -2