ctg 9π/4 = ctg (2.25π)

Так как ctg x = 1/tg x, то ctg (2.25π) = 1/tg (2.25π)

tg (π/4) = 1, поэтому tg (2.25π) = tg (π + π/4) = tg (π/4) = 1

Тогда ctg (2.25π) = 1/1 = 1

Ответ: ctg 9π/4 = 1