A rod of diameter D = 25 m and thermal conductivity of 60 W/m·K protrudes from a furnace with a wall temperature of 200ºC. The rod is welded to the furnace wall and is used as a hangar for instrumentation cables. To avoid damaging the cables, the surface temperature of last 100 m of the rod must be kept below 100ºC. The ambient air temperature is 25ºC and the convection coefficient is 15 W/m2K. A. Write the finite-difference equation for an internal node with x = 2 mm. B. Write the finite difference equation for the node at the fin tip.