sin a-cos b/sin b+cos a=sin b-cos a/sin a+cos b

по пропорции

(sina-cosb)*(sina+cosb)=(sinb+cosa)*(sinb-cosa)

sin^2a-cos^2d=sin^2b-cos^2a

представим sin^2a=1-cos^a

1-cos^2a-cos^2b=1-cos^2b-cos^2a

-cos^2a-cos^2b+cos^2b+cos^2a=1-1

0=0