Ctg(kx + b) ' = -k/(sin^2(kx+b))
ctg(x-1) ' = -1/sin^2(x-1)
либо воспользоваться другим способом (долгим):
ctg(x-1) = cos(x-1) / sin(x-1)
(a/b)' = (a' * b - a*b')/b^2
(cos(x-1) / sin(x-1))' = ( cos(x-1)' * sin(x-1) - cos(x-1)*sin(x-1)' ) / sin^2(x-1) = (-sin^2(x-1) - cos^2(x-1)) / sin^2(x-1) = -1/sin^2(x-1)