A thick-walled circular tube has inner radius r1 and outer radius r2. A gas heated to temperature T1 flows inside the tube and a gas at temperature T2 flows outside. The thermal conductivity k of the tube is constant. The heat transfer coefficients in the inner and outer tube flows are h1 and h2. Write the mathematical formulation of this heat conduction problem in order to determine the one-dimensional, stationary temperature distribution T(r) through the wall.