What is the maximum value of the sum of the numbers 36, 33, 30, 27, 24, ...?

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UPSC CDS-I 2025 (Elementary Mathematics) Official Paper (Held On: 13 Apr, 2025)
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  1. 240
  2. 237
  3. 234
  4. 231

Answer (Detailed Solution Below)

Option 3 : 234
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Detailed Solution

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Given:

The sequence is: 36, 33, 30, 27, 24, ...

We need to find the maximum value of the sum of the numbers.

This is an arithmetic progression (AP) where:

First term (a) = 36

Common difference (d) = 33 - 36 = -3

The sum of n terms of an AP is given by:

Formula used:

Sum (Sn) = \(\dfrac{n}{2}[2a + (n-1)d]\)

Calculation:

We need to find the maximum value of the sum. The AP ends when the last term becomes ≥ 0.

Last term (l) = a + (n-1)d

For the last term to be ≥ 0:

⇒ 36 + (n-1)(-3) ≥ 0

⇒ 36 - 3(n-1) ≥ 0

⇒ 36 - 3n + 3 ≥ 0

⇒ 39 - 3n ≥ 0

⇒ 3n ≤ 39

⇒ n ≤ 13

Thus, the sequence has a maximum of n = 13 terms.

Now, calculate the sum of these 13 terms:

S13 = \(\dfrac{13}{2}[2(36) + (13-1)(-3)]\)

⇒ S13 = \(\dfrac{13}{2}[72 + 12(-3)]\)

⇒ S13 = \(\dfrac{13}{2}[72 - 36]\)

⇒ S13 = \(\dfrac{13}{2}[36]\)

⇒ S13 = 13 × 18

⇒ S13 = 234

∴ The correct answer is option (3).

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