Design a furnace shaped like a long equilateral$-$triangular duct,heat is supplied from the base surface, whose emissivity is $0\mathrm{.}8,$ whilethe side surfaces, whose emissivities are $0\mathrm{.}5,$ are maintained at $500$ K$.$Neglecting the end effects. The temperature of the base surface is $543$K$.$Can you treat this geometry as a two$-$surface enclosure?