解1 由a为第三象限角且sina=-3/5
则cosa=-√1-sin²a=-4/5
即tana=sina/cosa=(-3/5)/(-4/5)=3/4
2 sin(a-π/2)+tan(7π-a)
=-sin(π/2-a)+tan(-a)
=-cosa-tana
=-(-4/5)-3/4
=4/5-3/4
=1/20
解1 由a为第三象限角且sina=-3/5
则cosa=-√1-sin²a=-4/5
即tana=sina/cosa=(-3/5)/(-4/5)=3/4
2 sin(a-π/2)+tan(7π-a)
=-sin(π/2-a)+tan(-a)
=-cosa-tana
=-(-4/5)-3/4
=4/5-3/4
=1/20