A sphere with radius 1 m has temperature 16C. It lies inside a concentric sphere with radius 2 m and temperature 21C. The temperature at a distance r from the common center of the spheres satisfies the differential equation below. If we let S = dT/dr, then S satisfies a first-order differential equation. Solve it to find an expression for the temperature T(r) between the spheres.

(d^2 T)/(d r^2) + 2/r (d T)/(d r) = 0