What is the antiderivative of 1/u?
Antiderivative or indefinite integral of a function f(u)=1/u is differentiable function
F such that F′(u) = f(u).
Now, we know that (ln u)′ = 1/u, so:
∫1/u du = ln u+c
We add the constant to every indefinite integral because derivative of constant is zero,
so (ln u+c)′ = 1/u