An arithmetic series has first term a a\,a and common difference ddd. The sum of the first n n\,n terms of the series is SnS_nSn.
By writing Sn S_n\,Sn out both forwards and in reverse, and adding the two expressions, prove that
Sn=12n[2a+(n−1)d]S_n = \tfrac{1}{2}n\left[2a + (n - 1)d\right]Sn=21n[2a+(n−1)d]
231 exam-style questions on WJEC A Level Maths 3.4 Sequences and Series (A-level only), covering 3.4.1 Sequences and Series (A-level only), 3.4.2 Sequences and Series (A-level only), 3.4.3 Sequences and Series (A-level only), 3.4.4 Sequences and Series (A-level only), 3.4.5 Sequences and Series (A-level only), 3.4.6 Sequences and Series (A-level only), and 3.4.7 Sequences and Series (A-level only). Each one has a worked solution and a mark scheme showing where the marks go.