Question

Помогите срочнооооо!!

Answer 1

$(ctg^2a-cos^2a) dfrac{1}{cos^2a} = \ \ =( dfrac{cos^2a}{sin^2a}-cos^2a)dfrac{1}{cos^2a} = \ \ =dfrac{cos^2a-sin^2acos^2a}{sin^2a} *dfrac{1}{cos^2a} = \ \ = dfrac{cos^2a(1-sin^2a)}{sin^2a}* dfrac{1}{cos^2a} = \ \ = dfrac{cos^4a}{sin^2a}* dfrac{1}{cos^2a} = \ \= dfrac{cos^2a} {sin^2a}= \ \=ctg^2a$