calculus – How to make a substitution in U with this result?

I saw this on a website. I work on Integrals and U-substitution does not make sense.

The initial problem was $ int $x / x + 1$ dx $

The U-Sub did u = x + 1, du = dx

so he gave me a $ int $u-1 / u of the …

How did my question become the numerator?