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NDA 2025
Exam
NDA
Year
2025
Subject
Mathematics
Chapter
Sequences and Series

NDA 2025 · Mathematics · Sequences and Series

If p , 1 , q are in AP and p , 2 , q are in GP, then which of the...

If pp, 11, qq are in AP and pp, 22, qq are in GP, then which of the following statements is/are correct?
I. pp, 44, qq are in HP.
II. 1p\dfrac{1}{p}, 14\dfrac{1}{4}, 1q\dfrac{1}{q} are in AP.
Select the answer using the code given below.

Options

  1. AI only
  2. BII only
  3. CBoth I and II
  4. DNeither I nor II

Correct answer

C. Both I and II

Step-by-step solution

Since pp, 11, qq are in AP, we have p+q=2(1)=2p + q = 2(1) = 2.

Since pp, 22, qq are in GP, we have pq=22=4pq = 2^2 = 4.

For pp, 44, qq to be in HP, the middle term must be the harmonic mean of pp and qq, which is given by 2pqp+q\dfrac{2pq}{p+q}.

Substituting the values of p+qp+q and pqpq, we get 2(4)2=4\dfrac{2(4)}{2} = 4.

Thus, pp, 44, qq are in HP. Statement I is correct.

By the definition of harmonic progression, if pp, 44, qq are in HP, their reciprocals 1p\dfrac{1}{p}, 14\dfrac{1}{4}, 1q\dfrac{1}{q} must be in AP.

Thus, Statement II is also correct.

Both statements I and II are correct.

Answer: Both I and II
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