y´= [(1- tg x)´ ^ (1+ tg x)] - [ (1 - tg x) (1 + tg x)´ ] / (1 + tg x)2 = [ - (1 + tg 2 x ) ( 1 + tg x)] - [ (1 - tg x ) ^ (1 + tg2 x)] / (1 + tgx)2 = [(-1 - tgx2)(1 + tgx)] - [(1 - tgx) (1 + tg2x)] / (1 + tgx)2 =[ (-1 -tgx - tg2 x - tg3 x) - ( 1 + tg2 x - tgx - tg 3 x)] / ( 1 + tgx)2 = - 2 tg2 x / (1 + tgx)2
