Question

ctg^2a-cos^2a=ctg^2a*cos^2a докажите тождество

Answer 1

$ctg^2a-cos^2a=frac{cos^2a}{sin^2a}-cos^2a=\ frac{cos^2a-cos^2a*sin^2a}{sin^2a}=frac{cos^2a(1-sin^2a)}{sin^2a}=frac{cos^2a*cos^2a}{sin^2a}=\ctg^2a*cos^2a$