====== Question 13 and 14, Exercise 4.3 ======
Solutions of Question 13 and 14 of Exercise 4.3 of Unit 04: Sequence and Series. This is unit of Model Textbook of Mathematics for Class XI published by National Book Foundation (NBF) as Federal Textbook Board, Islamabad, Pakistan. 

=====Question 13=====
Find $S_s$ for arithmetic series. $a_{1}=34$, $n=9$, $a_{n}=2$.

** Solution. **

Given $a_{1}=34$, $n=9$, $a_{n}=2$., then
\begin{align}
S_n&=\frac{n}{2}[a_1+a_n] \\
\implies S_{9}&=\frac{9}{2}[34+2]\\
&=162.
\end{align}
Hence $S_{9}=162$.
=====Question 14=====
Find $S_n$ for arithmetic series. $a_{1}=5$, $d=\frac{1}{2}$, $n=13$.

** Solution. **

Given: $a_{1}=5$, $d=\frac{1}{2}$, $n=13$,  then
\begin{align}
S_n&=\frac{n}{2}[2a_1+(n-1)d] \\
\implies S_{13}&=\frac{13}{2}\left[2(5)+(13-1)\times \frac{1}{2}\right]\\
&=\frac{13}{2}\left[10+6\right]\\
&=104.
\end{align}
Hence $S_{13}=104$. GOOD m(



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