(1)(sin a-3cos a)/(sin a+cos a)
=(tana-3)/(tana+1)
=-2/3
(2)(2sin^2 a-3cos^2 a)/(4sin^2 a-9cos^2 a)
=[2(tana)^2-3]/[(4(tana)^2-9]
=1/7
(3)本题问题错误.啊 不是吧怎么可能啊5cos^ a看看啊！！晕了是5cos^2 a(2)(2sin^2 a-3cos^2 a)/(4sin^2 a-9cos^2 a) =[2(tana)^2-3]/[(4(tana)^2-9]=5/7(3)4sin^2 a-3cos axsin a-5cos^ 2a=[4(sina)^2-3cosasina-5(cosa)^2]/[(sina)^2+(cosa)^2]=[4(tana)^2-3tana-5]/((tana)^2+1]=5/5=1