Question

The first and last terms of an AP are 1 and 10 respectively. If the total number of terms in the AP is 4, then $S_4$= .

• A. 20
• B. 11
• C. 22

Answer 1

The correct option is C 22

We know that sum to $n$ terms of an AP with $a$ and $l$ as the first and last terms is given by
$S_n=\frac{n}{2}(a+l).$
From question, we have $a=1$ and $l=10.$
Plugging in the values given, we have the sum of the terms of the AP is
$S_4=\frac{4}{2}(1+10)=2\times 11=22$.