A piece of insulation consists of a layer of material A and a layer of material B, each of thickness L, but with different thermal conductivities. Suppose that the insulation separates a reservoir at Th from a reservoir at Tl . Derive a formula for the steady-state temperature at the A/B interface. Explain the predictions of your formula for the cases kA>>kB and kA = kB. Hint: the heat fluxes through the two materials have to be the same.