====== Question 11, Exercise 4.6 ======
Solutions of Question 11 of Exercise 4.6 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 11=====
Find H.M. between $\dfrac{2}{3}$ and $\dfrac{4}{7}$.

** Solution. **

Here $a=\dfrac{2}{3}$ and $b=\dfrac{4}{7}$, therefore

\begin{align*}
\text{H.M.}&=\frac{2ab}{a+b} \\
&=\frac{2\times\frac{2}{3}\times\frac{4}{7}}{\frac{2}{3}+\frac{4}{7}} \\
&=\frac{16/21}{26/21} \\
&=\frac{8}{13} \\
\end{align*}
Hence $\dfrac{8}{13}$ is H.M between $\dfrac{2}{3}$ and $\dfrac{4}{7}$. GOOD






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