Was studying for my calculus exam, and came across something in a solution I don't quite understand.

How does the integral of 2x^2/1+x^2 turn into 2*the integral of 1-(1/1+x^2) (integral bounds are 0 to 1).

I understand how the 2 moves out of the integral, but I don't know how it turns into 1-(1/...). Is it some sort of rule I missed?