spherical shells Questions

Two concentric conducting thin spherical shells $A$ and $B$ having radii $r_A$ and $r_B$ ($r_B>r_A$) are charged to $Q_A$ and $-Q_B$ ($|Q_B|>|Q_A|$). The electrical field along a line (passing through the centre) is:

Three concentric spherical shells have radii $a,b$ and $c$ ($a<b<c$) and have surface charge densities $\sigma ,-\sigma$ and $\sigma$ respectively. If $V_A,V_B$ and $V_C$ denote the potentials of the three shells, then for $c=a+b$, we have: