Question

1/cos^2a-cos^2a-tg^2a

Answer 1

cos²a-ctg²a=cos²a- cos²a/sin²a=1/sin²a*(cos²a*sin²a-cos²a)=1/sin²a*cos²a*(sin²a-1)=  =1/sin²a * cos²a *(-cos²a)=-cos⁴a *1/sin²a