sin^2a +sinacosa/sin^2+1?是sin^2a +sinacosa/sin^2a+1吧?sin^2a +sinacosa/sin^2a+1=sin^2a +sinacosa/（2sin^2a+cos^2a) (因为1=sin^2a+cos^2a)=tan^2a+tana/2tan^2a+1 （分子,分母同除以cos^2a）=6/9=2/3...