NDA 2020 Maths · Mathematics · Sequences and Series · medium

In a harmonic progression the 4th term is 1/9 and the 7th term is 1/15.

Single correct option

In a harmonic progression the 4th term is

\dfrac19

and the 7th term is

\dfrac1{15}

. The 10th term is

Show answer and solution

Answer: (A) $\dfrac1{21}$

Reciprocals form an AP:

9

at the 4th place and

15

at the 7th, so

3d=6

and

d=2

. The 10th reciprocal is

15+3\cdot2=21

, so the term is

\dfrac1{21}

. Applying the AP formula to the terms themselves is the trap.

Original question written in the NDA 2020 Maths pattern and checked twice for the correct answer. Not an official question paper. How we write and check questions · Corrections policy · Report a mistake