f(α)=sin(π/2-α)cos(π/2+α)/sin(π+a)
=cosacosa/(-sina)
=(1-sin²a)/(-sina)
=sina-(1/sina)接着 已知f(a)=-4/5a为第二象限角 求sina tanaf(α)=sin(π/2-α)cos(π/2+α)/sin(π+a) =-cosasina/(-sina)=cosaf(a)=-4/5cosa=-4/5 sina=3/5所以tana=-3/4