Home Courses QCE Year 11 Maths C Arithmetic Sequences and Arithmetic Series (Free) Finding the Sum using a Formula ← Previous Topic Next Topic → TopicFinding the Sum using a Formula Topic Progress: The sum of n terms of an arithmetic sequence with a as its first term and l as its last term is given by \( \displaystyle{S}_{n} = \frac{n}{2}\Big[\text{2}{a} + {(n - 1)}d\Big] \).Practice Questions Show Solution Show Solution Show Solution Show Solution Show Solution Show Solution Show Solution Show Solution Show Solution Show Solution Show Solution Show Solution Show Solution Show Solution Show Solution Show Solution Show Solution Back to Lesson