Question
Insert two numbers between $\frac{1}{7}$ and $\frac{1}{13}$ so that the resulting sequence is a H.P. Solution:

Answer

Let the required numbers be $\frac{1}{\mathrm{H}_1}$ and $\frac{1}{\mathrm{H}_2}$.
$\therefore \frac{1}{7}, \frac{1}{\mathrm{H}_1}, \frac{1}{\mathrm{H}_2}, \frac{1}{13}$ are in H.P.
$\therefore 7, \mathrm{H}_1, \mathrm{H}_2$ and 13 are in A.P.
$\therefore \mathrm{t}_1=\mathrm{a}=7$ and $\mathrm{t}_4=\mathrm{a}+3 \mathrm{~d}=13$
$\therefore 7+3 d=13$
$\therefore 3 \mathrm{~d}=6$
$\therefore \mathrm{d}=2$
$\therefore \mathrm{H}_1=\mathrm{t}_2=\mathrm{a}+\mathrm{d}=7+2=9$
and $\mathrm{H}_2=\mathrm{t}_3=\mathrm{a}+2 \mathrm{~d}=7+2(2)=11$
$\therefore \frac{1}{9}$ and $\frac{1}{11}$ are the required numbers to be inserted between $\frac{1}{7}$ and $\frac{1}{13}$ so that the resulting sequence is a H.P.

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