Question

Доказать тождество;
$frac{cos2a}{sina*cosa+sin^2a} =ctga-1$

Answer 1

(cos²a-sin²a)/[sina(cosa+sina)]=(cosa-sina)(cosa+sina)/[sina(cosa+sina)]=
=(cosa-sina)/sina=cosa/sina-sina/sina=ctga-1
ctga-1=ctga-1