So I have two questions where I cannot find apply the antiderivative rule properly.

Q1

g(x) = 1/ 9(x^2 + 1)

The rule is 1 / (1+k^2 * x^2) = 1/k * tan^-1 * kx + C

I get 1/9 * tan^-1 * 9x + C

The answer for somereaso does not have a 9 in front of the x for the tan. How come? Isn't the K = 9 ?

Q2

Same problem except the question is 1 / 1+ 36x^2 and I get 1/36 * tan^-1 36x + C

Please help.