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What is the sum of the first 9 terms of an arithmetic progression if the first term is 7 and last term is 55?

A. 219 B. 279 C. 231 D. 137 Answer: Option B
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Solution(By Apex Team)

$\begin{aligned}S_n&=\frac{n}{2}(a+l)\\ S_9&=\frac{9}{2}(7+55)\\ S_9&=\frac{9}{2}\times62\\ S_9&=9\times31\\ &=279\end{aligned}$

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