1）(2sina+cosa)/(sina-3cosa) =(2tana+1)/(tana-3) (上下同除以cosa)=9/1=92）sinacosa-sin^2a=(sinacosa-sin^2a)/(sin^2a+cos^2a) (将1换成sin^2a+cos^2a)=(tana-tan^2a)/(tan^2a+1) (上下同除以cos^2a)=(4-16)/(1...