I tried searching everything but couldn't understand what formula or technique are they using to expand this term. I have attached image of my textbook for this problem. It's written there that we expand $(f(D))^{-1}$ in ascending power but I couldn't grasp it at all. Here is what they did: https://i.stack.imgur.com/c6DZi.jpg
$$ \frac{1}{D^2+2D-5} = -\frac{1}{5}\left[1 - (2D/5 + D^2/5)\right]^{-1} = -\frac{1}{5} \left[ 1 + ( 2D/5 + D^2/5) + (2D/5 + D^2/5)^2 + \dotsb \right] $$
Can you please explain how to expand like this? What formula/technique do they use?