Если sinx=-4/5, x∈(π;3π/2)
cosx = - √(1 - sin²x) = - √(1 - (-4/5)²) = - √(1 - 16/25) = - √(9/25) = - 3/5
tgx = sinx/cosx
tgx = - 4/5 : (- 3/5) = 4/3
ctgx = 1/tgx
ctgx = 3/4
