若sina+cosa/sina-cosa=2则4sin^2a-3sinacosa-5cos^2a=?tana=-1(-pai小于a小于pai）则a=

若sina+cosa/sina-cosa=2sina+cosa=2sina-2cosasina=3cosatana=3则4sin^2a-3sinacosa-5cos^2a=(4sin^2a-3sinacosa-5cos^2a)/(sin^2a+cos^2a),(分子分母同除以cos^2a)=(4tan^2a-3tana-5)/(tan^2+1)=(4*9-3*3-5)/(9+1)...