For being a guy who dreams of studying medicine one day, it hurts me to ask such a question because it makes me feel like a total idiot. Anyways... $1 - \frac{1}{1+x^2} = \frac{x^2}{1+x^2}$
I can go from the left part to the right part, but not the other way. I just don't understand what the trick is her. It's not obvious to me. I came a across this rewrite while doing integration by parts.
Any mental-math tip for me?