Question

Ctg^2a-cos^2a=cos^2/sin^2

Answer 1

cos a / sin a = ctg a

cos^2 a / sin^2 a = ctg^2 a

ctg^2 a - cos^2 a = ctg^2 a

cos^2 a = 0

cos a = 0

a = pi/2 + n * pi; n принадлежит натральным