(cota/2-tana/2)/(cota/2+tana/2)
=(cosa/2/sina/2-sina/2/cosa/2)/(cosa/2/sina/2+sina/2/cosa/2)
=(cos²a/2-sin²a/2)/(cos²a/2+sin²a/2)
=(cos²a/2-sin²a/2)
=cosa