(1-cos^2 x) ctg^2 x + (1-sin^2 x) tg^2 x = sin^2 x * ctg^2 x + cos^2 x * tg^2 x = cos^2 x + sin^2 x = 1
(1-cos^2 x) ctg^2 x + (1-sin^2 x) tg^2 x = sin^2 x * ctg^2 x + cos^2 x * tg^2 x = cos^2 x + sin^2 x = 1