Which of the following expressions gives the sum of the first terms of an arithmetic…

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

Problem

Which of the following expressions gives the sum

S_n

of the first

n

terms of an arithmetic progression when only the first term

a

and the last term

l

are known? (a)

\dfrac{n}{2}\,[2a + nd]

(b)

n\,[2a + (n-1)d]

(c)

\dfrac{n}{2}\,[2a + (n-1)d]

(d)

\dfrac{n}{2}\,[a + (n-1)d]