Question : What is the sum of the first 9 terms of an arithmetic progression, if the first term is 7 and the last term is 55?
Option 1: 219
Option 2: 137
Option 3: 231
Option 4: 279
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Correct Answer: 279
Solution :
Given: The first term is 7 and the last term is 55.
Using the formula, S
9
= $\frac{n}{2}(a+l)$
Where $a$ is the first term, $l$ is the last term of the A.P., and n is the number of terms.
By putting the value of 1st and last term,
⇒ S
9
= $\frac{9}{2}$(7 + 55)
⇒ S
9
= $\frac{9}{2}$ × 62
$\therefore$ S
9
= 9 × 31 = 279
Hence, the correct answer is 279.
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