已知f(a)=sin(π-a)cos(2π-a)tan(-π+3π/2)/tan(π/2)sin(-π-a)若a是第三象

a是第三象限角
cos(a-3π/2)=1/5
cos(a-3π/2)=cos(-2π+(π/2+a))=cos(π/2+a)=cos(π-（π/2-a))=-cos(π/2-a)=-sina=1/5
sina=-1/5
cosa=-根号(1-sin^2a)=-2根号6 /5
f(a)=sin(π-a)cos(2π-a)tan(-π+3π/2) / {tan(π/2)sin(-π-a)}
=sinacosa tan(-π+π+π/2) / {tan(π/2)[-sin(-a)]}
=sinacosa tan(π/2) / {tan(π/2)sina}
=cosa
=-2根号6 /5