SHOW HOW THE GOVERNING EQUATION $($mathmatical formulation was found$)$ show all steps!!

Example $5-5T=T(r,t)$ for Hollow Sphere

A hollow sphere of inner radius a and outer radius $b$ is initially at temperature $F(r)($Fig$.5-3).$ For $t>0,$ the inner and outer surfaces are maintained at zero temperature.

Figure $5-3$ Problem description for Example $5-5.$

The mathematical formulation of the problem is given as

$\frac{de{l}^{2}T}{del{r}^2}+\frac{2}{r}\frac{\text{d}\text{e}\text{l}\text{T}}{\text{d}\text{e}\text{l}\text{r}}=\frac{1}{}\frac{\text{d}\text{e}\text{l}\text{T}}{\text{d}\text{e}\text{l}\text{t}},in,a0$