Can somebody explain me this?

[Kruger]

I postet the formula below (attachment). I don't know how we can get from the first step to the second. I mean why does that n fall "out" of the square root?

 

[mathman]

Step 1a, let v=u+n², then lower limit of v is n²
Step 1b, let u=v

I presume w is independent of u.