Question

докажите
 тождество
 cos^2a-sin^2a=2cos^2*tga/tg 2a

Answer 1

$cos^2a-sin^2a=2cos^2a*frac{tga}{tg 2a};$
$cos2a=2cos^2a*frac{sinacos2a}{cosasin2a};$
$cos2a=frac{2cos^2asinacos2a}{2cosasinacosa};$
$cos2a= cos2a.$