\frac{\partial C_1}{\partial t}=\frac{\partial}{\partial x}[D_1 \frac{\partial C_1}{\partial x} -\frac{C_1}{C}[D_1 \frac{\partial C_1}{\partial x} + D_2\frac{\partial C_2}{\partial x}]] .
As stated before, \textstyle \frac{\partial C_1}{\partial x} = -\frac{\partial C_2}{\partial x} which gives: .