P 4.15
a) ~E(z0) = { ρ0δ/ε0 [1− exp(−2a/δ)] ẑ if z0 > a, −ρ0δ/ε0 [1− exp(−2a/δ)] ẑ if z0 < −a }
b) ~E(z0) = { ρ0δ/ε0 ẑ if z0 > a, −ρ0δ/ε0 ẑ if z0 < −a }
c) V (z) = { V0 if −a ≤ z ≤ a, V0 − ρ0δ/ε0(z − a) if z > a, V0 + ρ0δ/ε0(z + a) if z < −a }