Question

ctg^2a-1=cos2a/sin^2a

Answer 1

Ctg^2a-1=cos2a/sin^2aсos^2x / sin^2x - 1 - cos2x/sin^2x = 0
(cos^2x - sin^2x - cos2x) /  [email protected] = 0
(cos2x - cos2x) / sin^2x = 0
0 / si^2x = 0 
2x ≠ 0
2x ≠ πn, n∈Z
x ≠ (πn)/2, n∈Z
Ответ х - любое число. кроме  x = (πn)/2, n∈Z