sec = 1 / cos; tan = sen / cos
[(1 / cos x) + (sen x / cos x) ] = 1 / [(1 / cos x - sen x / cos x)]
( 1 + sen x / cos x ) = 1 : (1 - sen x / cos x)
(1 + sen x / cos x) = (cos x / 1 - sen x)
(1 + sen x) (1 - sen x) = cos 2 x
1 - sen 2 x = cos 2 x
cos 2 x + sen 2 x = 1 (Ecuación fundamental de la trigonometría)
