Which of the following is the correct expression for the sum of the first terms of an…

MATHEMATICS · Single correct · A Level · Mathematics · Sequences and series · Sequences and series

Problem

Which of the following is the correct expression for the sum SnS_n of the first nn terms of an arithmetic progression whose first term is aa and common difference is dd? (a) Sn=n2[a+(n−1)d]\displaystyle S_n = \frac{n}{2}\bigl[a + (n-1)d\bigr] (b) Sn=n2[2a+nd]\displaystyle S_n = \frac{n}{2}\bigl[2a + nd\bigr] (c) Sn=n2[2a+(n−1)d]\displaystyle S_n = \frac{n}{2}\bigl[2a + (n-1)d\bigr] (d) Sn=n[a+(n−1)d]\displaystyle S_n = n\bigl[a + (n-1)d\bigr]

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