2．The sum of the first \(p\) terms of an arithmetic series is \(q\) and the sum of the first \(q\) terms of the same arithmetic series is \(p\) ，where \(p\) and \(q\) are positive integers and \(p \neq q\) ． Giving simplified answers in terms of \(p\) and \(q\) ，find
（a）the common difference of the terms in this series，
（b）the first term of the series，
（c）the sum of the first \(( p + q )\) terms of the series．