NDA 2018
- Exam
- NDA
- Year
- 2018
- Subject
- Mathematics
- Chapter
- Sequences and Series
NDA 2018 · Mathematics · Sequences and Series
If x, (3)/(2), z are in AP; x, 3, z are in GP; then which one of...
If are in AP; are in GP; then which one of the following will be in HP?
Options
- Ax, 6, z
- Bx, 4, z
- Cx, 2, z
- Dx, 1, z
Correct answer
A. x, 6, z
Step-by-step solution
a, b, care in If $a, b, c$ are in G.P. $b^2=\mathrm{ac}\begin{aligned}
& \therefore 3^2=\mathrm{xz} \\
& \Rightarrow 9=\mathrm{xz}...(2)
\end{aligned}$
If $x, 6, z$ are in H.P. $\frac{2}{6}=\frac{1}{x}+\frac{1}{z}\begin{aligned}
& \qquad\left(\because \frac{2}{b}=\frac{1}{a}+\frac{1}{c} \text{, if } a, b, c \text{ are is H.P. }\right) \\
& \Rightarrow \frac{1}{3}=\frac{z+x}{x z}=\frac{3}{9}=\frac{1}{3}(\operatorname{from}(1)(2)) \\
& \text{ L.H.S. }=\text{ R.H.S. }
\end{aligned}$$$