(sin^3a+cosa)/(sin^3a+sina)分子分母同除cosa
=(tanasin^2a+1)/(tanasina+tana)
=(2sin^2a+sin^2a+cos^2a)/(2sin^2a+2sin^2a+2cos^22a)
=(3sin^2a+cos^2a)/(4sin^2a+2cos^2a)分子分母同除cos^2a
=(3tan^2a+1)/(4tan^2a+2)
=(3*4+1)/(4*4+2)
=13/18