Question

An AP has four terms. If its first and last terms are 1 and 5 respectively, then its sum is

• A. 21
• B. 24
• C. 6
• D. 12

Answer 1

The correct option is D 12

We know that sum to n terms of an AP is $\frac{n}{2}(a+l)$, where a and l are the first and last terms respectively, and n is the number of terms.

Given that an AP has four terms, first and last terms are 1 and 5 respectively.
$n=4,a=1,l=5$
$S_n=\frac{n(a+l)}{2}$
$S_n=\frac{4(1+5)}{2}$
$S_n=12$