sin^2 a*tana+cos^2*cota+2sina*cosa
=sin^2 a*sin a/cosa+cos^2a*cosa/sin a+2sina*cosa
=(sin^4 a+cos^4a)/sina*cosa+2sina*cosa
=(sin^2 a+cos^2a)^2/sina*cosa-2sina*cosa+2sina*cosa
=1/sina*cosa
=2/sin2a
sin^2 a*tana+cos^2*cota+2sina*cosa
=sin^2 a*sin a/cosa+cos^2a*cosa/sin a+2sina*cosa
=(sin^4 a+cos^4a)/sina*cosa+2sina*cosa
=(sin^2 a+cos^2a)^2/sina*cosa-2sina*cosa+2sina*cosa
=1/sina*cosa
=2/sin2a