A parallel plate capacitor has plates of area $A$ separated by distance 'd' between them. It is filled with a dielectric which has a dielectric constant that varies as $k(x)=K(1+\alpha x)$ where ' $x$ ' is the distance measured from one of the plates. If $(\alpha \mathrm{d}) \ll 1$, the total capacitance of the system is best given by the expression