well u see the explanation given in resnic and halliday is not that clear, read the book "lectures on physics"- by Richard. Feynman.
he giv's it like this
in a conducting metal plate charges are spread on both sides equally, so electric field at each side is given by ?/2? 0---(1) (according to gauss law?E.F due to non conducting insulator plate has charge on one side only?check in resnic and halliday) ---so for total :- ?/? 0, for a cap'r since the opp charged plates are kept close together charge only exist on one side. Therefore E.F is
same as (1) i.e ?/2? for each plate on one side . thus total E.F due to cap?r is
?/2? 0 + ?/2? 0 = ?/? 0 ??
H.C Verma is also has simplified explanation than "lectures on physics"....