1/sin2a+1/tan2a+1/sina=1/sin2a+cos2a/sin2a+1/sina=(1+cos2a)/sin2a+1/sina=(1+2cosa^2-1)/2sinacosa+1/sina=cosa/sina+1/sina=(1+cosa)/sina=(1+2cosa/2^2-1)/2(sina/2)(cosa/2)=(cosa/2)/(sina/2)=1/(tana/2)