1. The problem statement, all variables and given/known data This problem has been stumping me for days now, I'm sure i'm missing something simple as it's only worth a small number of marks on the coursework. Any help would be appreciated. I've been asked to re-express the equation of hydrostatic equilibrium: dP/dr = - Gm/4πr4 dm/dr as, -Gm/4πr4 dm/dr = -d/dr (Gm2/8πr4) - Gm2/2πr5 2. Relevant equations n/a 3. The attempt at a solution The most obvious thing seemed to try to differentiate with respect to r using product rule. This however leaves me with the wrong answers and an imbalance in my equation. I've also tried integrating by parts but i don't believe this is the correct way to tackle the problem as you can't get the r5 by integration, only by differentiation. Any help would be appreciated, I'm sure i'm missing something really simple. Thanks, John.