Solve a source$-$free $1$D convection$-$diffusion problem shown in the figure below using upwind differencing to show that the Cartesian discretised equations can be obtained from the unstructured grid formulation. Consider equal grid spacing and $A=1\mathrm{.}0$ for all interfaces.

$\frac{d}{dx}(u)=\frac{d}{dx}(\frac{d}{dx})$