Question

Помогите пожалуйста !

Answer 1

$ctga=sqrt2-1quad to quad tga=frac{1}{tga}=frac{1}{sqrt2-1}=sqrt2+1\\cos2a= frac{ctga-tga}{ctga+tga} = frac{sqrt2-1-(sqrt2+1)}{sqrt2-1+sqrt2+1} =frac{-2}{2sqrt2}=-frac{1}{sqrt2}=-frac{sqrt2}{2}$