sec α = 1/cos α
1) cos α . sec α = 1 ----> cos α . 1/cos α = 1 ------> 1 = 1
cotg α = 1/tg α
2) tg α . cotg α = 1 ----> tg α . 1/tg α = 1 -----> 1 = 1
cotg α = cos α /sen α
3) sen α . cotg α = cos α ---> sen α . cos α /sen α = cos α ----> cos α = cos α