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What is the sum of the first 11 terms of an arithmetic progression if the 3rd term is -1 and the 8th term is 19?

A. 204 B. 121 C. 225 D. 104 Answer: Option B
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Solution(By Apex Team)

$\begin{aligned}&T_3=a+2d=-1\ldots.\\ &T_8=a+7d=19\ldots(2)\\ &\text{ on solving }(1)\text{ and }(2)\\ &d=4\&a=-9\\ &S_n=\frac{n}{2}[2a+(n-1)d]\\ &S_{11}=\frac{11}{2}[2(-9)+(11-1)(4)]\\ &S_{11}=\frac{11}{2}[(-18)+(40)]\\ &S_{11}=\frac{11}{2}[22]\\ &S_{11}=121\end{aligned}$

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