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Two walls of thicknesses ${\mathrm{d}}_{1}$ and ${\mathrm{d}}_{2}$ and thermal conductivities ${\mathrm{k}}_{1}$ and ${\mathrm{k}}_{2}$ are in contact. In the steady state, if the temperatures at the outer surfaces ${\mathrm{T}}_{1}$ and ${\mathrm{T}}_{2}$ are and , the temperature at the common wall is

(a) $\frac{{\mathrm{k}}_{1}{\mathrm{T}}_{1}{\mathrm{d}}_{2}+{\mathrm{k}}_{2}{\mathrm{T}}_{2}{\mathrm{d}}_{1}}{{\mathrm{k}}_{1}{\mathrm{d}}_{2}+{\mathrm{k}}_{2}{\mathrm{d}}_{1}}$                      (b)$\frac{{\mathrm{k}}_{1}{\mathrm{T}}_{1}+{\mathrm{k}}_{2}{\mathrm{d}}_{2}}{{\mathrm{d}}_{1}+{\mathrm{d}}_{2}}$
(c) $\left(\frac{{\mathrm{k}}_{1}{\mathrm{d}}_{1}+{\mathrm{k}}_{2}{\mathrm{d}}_{2}}{{\mathrm{T}}_{1}+{\mathrm{T}}_{2}}\right){\mathrm{T}}_{1}{\mathrm{T}}_{2}$                    (d) $\frac{{\mathrm{k}}_{1}{\mathrm{d}}_{1}{\mathrm{T}}_{1}+{\mathrm{k}}_{2}{\mathrm{d}}_{2}{\mathrm{T}}_{2}}{{\mathrm{k}}_{1}{\mathrm{d}}_{1}+{\mathrm{k}}_{2}{\mathrm{d}}_{2}}$