We have $S_n$ = pn + $qn^2$, where $S_n$ be the sum of n terms. $S_1$ = p + q (sum of 1 term i.e., first term) $S_2$ = 2p + 4q (sum of 2 terms i.e., sum of first term & second term) Also, $S_2$ = $a_1 + a_2$ ∴ 2p + 4q = p + q + $a_2$ ⇒ p + 3q = $a_2$ Thus, common difference d = $a_2 – a_1$ = p + 3q – p –q = 2q