Упростите плиз! cos^2a +ctg^2a-1/sin^2a=?

Question

Упростите плиз!

cos^2a +ctg^2a-1/sin^2a=?

Answer 1

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