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 BShow Answer
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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