Sequence and Series - Arithmetic Progression

Let \(S_n\) be the sum of the first \(n\) terms of an arithmetic sequence, whose first three terms are \(u_1, u_2\) and \(u_3\). It is known that \(S_1 = 7\), and \(S_2 = 18\).[6]

1. Write down \(u_1\).
2. Calculate the common difference of the sequence.
3. Calculate \(u_4\).