Mathematics

Problem

Which of the following gives the sum

S_n

of the first

n

terms of an arithmetic progression whose first term is

a

and last term is

l

? a)

S_n = \dfrac{n}{2}\,(l - a)

S_n = \dfrac{n}{2}\,(a + l)

S_n = n(a + l)

S_n = \dfrac{a + l}{2n}