cos(π/2+α )=2sin(α -π/2）即-sina=-2cosatana=2sin^3(π-α )+cos(α -π)/5cos（5π/2-α ）+3sin（7π/2-α ）=(sin^3a-cosa)/(5sina-3cosa) (上下同除cosa)=(sin^2a*tana-1)/(5tana-3)=(2sin^2a-1)/7=1/7*(s...