f(α)=sin(π-α)cos(2π-α)tan(-α+π)/sin(π+α)tan(2π-α)
=sinacosa(-tana)/(-sina)(-tana)
=-(sinacosatana/sinatana)
=cosa
cos（α-3π/2）=1/5
=cosacos3π/2+sinasin3π/2
=-sina=1/5
sina=-1/5
a在第三象限
cosa=负的五分之二倍根六
a=1920°
cos1920°=cos(11π-60°)=cos(π-60°)=-cos60°=-1/2