Ответы и объяснения

2013-01-28T18:25:59+04:00

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

cos^2a/sin^2a - cos^2a = cos^2a/Sin^2a * cos^2a

(cos^2a - cos^2a * sin^2a)/sin^2a = cos^4a/sin^2a

cos^2a - cos^2a * sin^2a = cos^4a

cos^2a = cos^2a(sin^2a + cos^2a)

1 = 1