（1）化简f(a)
f(a)=sin(兀-a)cos(2兀-a)tan(-a-3兀/2)/cot(-a-兀)sin(-a-兀)
=sina*cosa*cota/(-cota)*sina
=-cosa.
（2）若cos(a-3兀/2)=1/5,求f(a)的值
cos(a-3兀/2)=-sina=1/5,
f(a)=-cosa=±4√6/5.
（3）若a=-1860度,求f(a)的值
f(a)=-cos(-1860°)=-cos(-360°*5-60°)
=-cos(-60°)=-cos60°=-1/2.