1. The problem statement, all variables and given/known data E=(1/4πε0)(Q/r^2) for R<r<2R 2. Relevant equations U= integral (2R,R) ( (ε0 E^2)/2*4πr^2 dr 3. The attempt at a solution I have no idea where the U-formula comes from. Any help would be appreciated. I added some pictures so that it could be easier to understand.