(sina+cosa)/(sina-cosa)=3
sina+cosa=3sina-3cosa
2sina=4cosa
tana=2
1/（sin²a-sinacosa-cos²a）=(sin^2a+cos^2a)/(sin^2a-sinacosa-cos^2a)
=(tan^2a+1)/(tan^2-tana-1)
=(4+1)/(4-2-1)
=5