An arithmetic progression starts with term $2$ and has common difference $d$. Write the sum of the first $n$ terms as $S_n$.
(a)[4]
It is stated that $(S_2 - 1)$, $S_4$, $S_9$ are the initial three terms of a second arithmetic progression. Determine the value of $d$.
(b)[4]
Hence determine the difference between the values of the 15th terms of the two arithmetic progressions.
Worked solution & mark scheme
This 8-mark question has a full step-by-step worked solution and mark scheme. One marking point: “Try to rewrite each of $S_2-1$, $S_4$, $S_9$ using $d$” …