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NDA 2026
Exam
NDA
Year
2026
Subject
Mathematics
Chapter
Sequences and Series

NDA 2026 · Mathematics · Sequences and Series

If p , g_1 , g_2 and q are in GP and m is the arithmetic mean of p...

If pp, g1g_1, g2g_2 and qq are in GP and mm is the arithmetic mean of pp and qq, then g12g2+g22g1\dfrac{g_1^2}{g_2}+\dfrac{g_2^2}{g_1} is equal to

Options

  1. Amm
  2. B2m2m
  3. C11
  4. D12\dfrac{1}{2}

Correct answer

B. 2m2m

Step-by-step solution

Given pp, g1g_1, g2g_2, qq are in GP.

Let the common ratio of the GP be rr.

Then g1=prg_1 = pr, g2=pr2g_2 = pr^2, and q=pr3q = pr^3.

The arithmetic mean of pp and qq is mm, so m=p+q2⇒p+q=2mm = \dfrac{p + q}{2} \Rightarrow p + q = 2m.

Consider the expression g12g2+g22g1\dfrac{g_1^2}{g_2} + \dfrac{g_2^2}{g_1}.

Substituting the values of g1g_1 and g2g_2:

g12g2+g22g1=(pr)2pr2+(pr2)2pr\dfrac{g_1^2}{g_2} + \dfrac{g_2^2}{g_1} = \dfrac{(pr)^2}{pr^2} + \dfrac{(pr^2)^2}{pr}

=p2r2pr2+p2r4pr= \dfrac{p^2r^2}{pr^2} + \dfrac{p^2r^4}{pr}

=p+pr3= p + pr^3

Since q=pr3q = pr^3, we get:

=p+q= p + q

Substituting p+q=2mp + q = 2m:

g12g2+g22g1=2m\dfrac{g_1^2}{g_2} + \dfrac{g_2^2}{g_1} = 2m

Answer: 2m2m
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