How do they know that √ 2/n is less than b-a in the proof in the image?

Sorry if this is in the wrong section, it's week one of my first pure maths course so I'm not really sure what we are studying. The module name is just intro to pure maths

I understand a different proof for showing that between any two real numbers there is an irrational so I don't need a proof, I was just wondering if anyone could help with the step that has me confused in this specific proof?

The proof is attached as an image

Thanks

 

nvm i worked it out, sorry for bothering anyone who reads this