解
(sina+cosa)/(sina-cosa)——分子分母同时除以cosa
=(tana+1)/(tana-1)
=(3+1)/(3-1)
=2
sinacosa
=(sinacosa)/(sin²a+cos²a)——原式除以cos²a+sin²a=1
=(tana)/(tan²a+1)——分子分母同时除以cos²a
=3/(9+1)
=3/10