Hey. I always assumed that when calculating with infinitesimal amounts, like dV = infinitesimal change in volume, you can play around with them the same as you'd do with regular numbers. For example, you if you multiply dV/dt with dr/dr, you have dV/dt=dV/dr * dr/dt. And so on. What exactly are the restrictions? I'm having my first calculus exam at university level tomorrow, and I thought I could use some help. Also: Do you guys have any tips on general strategies for solving hard/unexpected Calculus problems?