What is the sum of the first 12 terms of an arithmetic progression if the 3rd term is -13 and the 6th term is -4?
A. 67 B. 45 C. -30 D. -48 Answer: Option CShow Answer
Solution(By Apex Team)
$\begin{aligned}&T_3=a+2d=-13\ldots\ldots\text{ (1) }\\
&T_6=a+5d=-4\ldots\text{ (2) }\\
&\text{ on solving (1) and }\\
&d=3\&a=-19\\
&S_n=\frac{n}{2}[2a+(n-1)d]\\
&S_{12}=\frac{12}{2}[2(-19)+(12-1)(3)]\\
&S_{12}=(6)[-38+33]\\
&S_{12}=-30\end{aligned}$
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