[tex]C = \frac{S\cdot \epsilon_{0}\cdot \epsilon_{r}}{d} \ \ /\cdot d\\\\S\cdot \epsilon_{0}\cdot \epsilon_{r} = C\cdot d \ \ /:(S\cdot \epsilon_{0})\\\\\epsilon_{r} = \frac{C\cdot d}{S\cdot \epsilon_{0}}[/tex]
