是cos(a-π/2)/(sin(5π/2+a)*sin（a-2π）*cos（2π-a）)么?
cos(a-π/2)=-sin(a); sin(5π/2+a)=sin(π/2+a)=cos(a); sin（a-2π）=sin(a); cos（2π-a）=cos(a)
剩下的就好算了