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

NDA 2025 · Mathematics · Sequences and Series

For the following two (02) items: Let (6 + 10 + 14 + up to m terms)...

For the following two (02) items:
Let (6+10+14+… up to m terms)=(1+3+5+7+… up to n terms)(6 + 10 + 14 + \ldots\text{ up to }m\text{ terms}) = (1 + 3 + 5 + 7 + \ldots\text{ up to }n\text{ terms}), where m<25m < 25 and n<25n < 25.

What is the relation between mm and nn?

Options

  1. An2=m(m+1)n^2 = m(m+1)
  2. Bn2=m(m+2)n^2 = m(m+2)
  3. Cn2=2m(m+1)n^2 = 2m(m+1)
  4. Dn2=2m(m+2)n^2 = 2m(m+2)

Correct answer

D. n2=2m(m+2)n^2 = 2m(m+2)

Step-by-step solution

The first series is an arithmetic progression with first term a=6a = 6 and common difference d=4d = 4. The sum up to mm terms is given by:

Sm=m2[2(6)+(m−1)4]S_m = \dfrac{m}{2}[2(6) + (m-1)4]

Sm=m2[12+4m−4]=m2[4m+8]=2m(m+2)S_m = \dfrac{m}{2}[12 + 4m - 4] = \dfrac{m}{2}[4m + 8] = 2m(m + 2)

The second series is an arithmetic progression with first term a=1a = 1 and common difference d=2d = 2 (which is the sum of the first nn odd natural numbers). The sum up to nn terms is given by:

Sn=n2[2(1)+(n−1)2]=n2S_n = \dfrac{n}{2}[2(1) + (n-1)2] = n^2

Equating the two sums, we get:

n2=2m(m+2)n^2 = 2m(m + 2)
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