(1)f(a)=sin(a-π/2)cos(3π/2-a)tan(7π-a) / tan(-a-5π)sin(a-3π) =-cosa(-sina)(-tana)/[(-tana)(-sina)] =-cosa (2)tan(a-3π/2)=-2 则：cota=2 tana=1/2 sina=1/2cosa sin^2a=1/4cos^2a 1-cos^2a=1/4cos^2a cos^2a=4/5 cosa=2√5/5或-2√5/5 故f(a)=-cosa=-2√5/5或2√5/5