已知f(a)=[sin(-a+π/2)*cos (3π/2-a)*tan(a+5π)]/[tan(-a-π)*sin(a

1.f(a)=[cosa*(-sina)*tana]/[-tana*(-sina)]
=-cosa
2.cos(a-3π/2)=-sina=-1/5
cos^2a=24/25 a是第三项象限角
cosa=-2√6/5
f(a)=2√6/5
3.f(a)=-cos(-31π/3)=-cos(-10π-π/3)=-cosπ/3=-1/2