左边=sin^2a+cos^2a+2sinacosa-cos^2a+sin^2a/sin^2a+cos^2a+2sinacosa+cos^2a-sin^2a
=2sin^2a+2sinacosa/2cos^2a+2sinacosa
=2sina(sina+cosa)/2cosa(cosa+sina)
=2sina/2cosa
=tana=右边
所以1+sin2-cos2a/1+sin2a+cos2a=tana