Problem $3.$

Consider a spherical capacitor consisting of concentric conducting spheres of radii $\backslash ($ a $\backslash )$ and $\backslash ($ b $\backslash ).$ If the region between the spheres is vacuum, then the capacitance is $\backslash (4\backslash$pi $\backslash$varepsilon$\_\{0\}$ a b $/($b$-$a$)\backslash ).$

$($a$)$ What is the capacitance if the volume outside of inner sphere is filled up to radius $\backslash ($ c$($a