sina+cosa/sina-cosa=2
除以cosa得
(tana+1)/(tana-1)=2
tana+1=2tana-2
tana=3
[4sin^2a-3sinacosa-5cos^2a]/(sin²a+cos²a)
除以 cos²a得
=[4tan²a-3tana-5]/(tan²a+1)
带入 tana=3得
=(4*9-3*3-5)/(9+1)
=22/10
=11/5