f(a)=sin(π-α)cos(2π-α)tan(-α+π)/sin(π+α)
=sinacosa(-tana)/(-sina)
=sinacosasina/cosa *1/sina
=sina

cos(a-3π/2)
=cos(3π/2-a)
=-sina=1/5
sina=-1/5

所以
f(a)=-1/5