Докажите тождество: 1+(сtg^2a-tg^2a)*cos^2a=ctd^2a

[latex]1+ (ctg^2a-tg^2a)*cos^2a=ctg^2a\\\\1+ (\frac{cos^2a}{sin^2a}- \frac{sin^2a}{cos^2a})*cos^2a=ctg^2a\\\\1+ \frac{cos^4a-sin^4a}{sin^2a*cos^2a}*cos^2a =ctg^2a\\\\1+ \frac{(cos^2a-sin^2a)(cos^2a+sin^2a)}{sin^2a}=ctg^2a\\\\ \frac{sin^2a+(cos^2a-sin^2a)*1}{sin^2a}=ctg^2a \\\\ \frac{sin^2a+cos^2a-sin^2a}{sin^2a}=ctg^2a\\\\ \frac{cos^2a}{sin^2a}=ctg^2a\\\\ctg^2a=ctg^2a [/latex]