Докажите тождество :
sin^2a+cos^2a/tg^2a*cos^2a - cos^2a/1-cos^2a = 1

sin^2a+cos^2a/tg^2a*cos^2a - cos^2a/1-cos^2a =

1/(sin^2a/cos^2a *cos^2a)- cos^2a/sin^2a =

1/sin^2a  - cos^2a/sin^2a =(1  - cos^2a)/sin^2a=

sin^2a/sin^2a =1