Solve the foregoing problem for the case of two concentric spheres of radii `R_1` and `R_2` and temperatures `T_1` and `T_2`.

Explanation

In equilibrium `4 pi r^2 k (dT)/(dr) = - A = constant`
`T = B + (A)/(4 pi k) (1)/( r)`
Using `T = T_1` when `r = R_1` and `T = T_2` when `r = R_2`,
`T = T_1 + (T_2 - T_1)/((1)/(R_2) -(1)/(R_1)) ((1)/(r) -(1)/(R_1))`.