Question

ctg²a-ctg²a*cos²a-cos²a

Answer 1

$ctg^2x-ctg^2xcdot cos^2x-cos^2x=ctg^2x(1-cos^2x)-cos^2x=\\=frac{cos^2x}{sin^2x}cdot sin^2x-cos^2x=cos^2x-cos^2x=0$

Answer 2

формулы ctg=cos/sin 1/sin^2=1+ctg^2
cos^2/sin^2-(cos^2/sin^2)cos^2-cos^2=cos^2(1/sin^2-cos^2/sin^2-1)=
=cos^2(1+ctg^2-ctg^2-1)=cos^2(0)=0