Question

Докажите тождество:
cos^2a(1+tg^2a) - sin^2a = cos^2a

Answer 1

cos^2a(1+tg^2a) - sin^2a = cos^2a 
cos^2a(1+tg^2a) - sin^2a =cos^2a+cos^2a*sin^2acos^2a+sin^2a=cos^2a+sin^2a+sin^2a=1+sin^2a=sin^2a