1 + (cos^4t+sin^2tcos^2t) / sin^2t=1/sin^2t

1 + (cos^4t+sin^2tcos^2t) / sin^2t=1/sin^2t 1 + cos^2t (cos^2t+sin^2t) / sin^2t=1/sin^2t 1 + cos^2t/sin^2t=1/sin^2t (cos^2t+sin^2t) / sin^2t = 1/sin^2t 1/sin^2t = 1/sin^2t 1=1 тождество доказано.