# Help With Partial Derivatives and Infinite Sums

 P: 2 I'm working on a calculus project and I can't seem to work through this next part... I need to substitute equation (2) into equation (1): (1): r$\frac{\partial}{\partial r}$(r$\frac{\partial T}{\partial r}$)+$\frac{\partial ^{2}T}{\partial\Theta^{2}}$=0 (2): $\frac{T-T_{0}}{T_{0}}$=A$_{0}$+$\sum$ from n=1 to infinity of ($\frac{r}{R}$)$^{n}$(A$_{n}$cos(n$\Theta$)+B$_{n}$sin(n$\Theta$)) I know I have to solve for T in the second equation and then substitute but I don't really know the rules for infinite sums... The whole point of this is to prove that equation (2) is a solution to equation (1). Any help or advice would be appreciated!