in x = a/c
cos x = b/c
tg x = a/b=sinx/cos x
ctg x = b/a = cos x/sin x
sin ( - ) = sin
sin ( /2 - ) = cos
cos ( /2 - ) = sin
cos ( + 2 k) = cos
sin ( + 2 k) = sin
tg ( + k) = tg
ctg ( + k) = ctg
sin² + cos² =1
ctg = cos / sin , n, n Z
tg ctg = 1, ( n)/2, n Z
1+tg² = 1/cos² , (2n+1)/2
1+ ctg² =1/sin² , n ЭЛЕМЕНТАРНО ОТВЕТ В