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Solution


Given:
    • The sum of first 11 terms of an arithmetic progression = 275
    • First term and common difference are positive integers

To find:
    • The sum of the first 12 terms

Approach and Working:
    • Let us assume that the first term of the arithmetic progression is ‘a’ and common difference is ‘d’
    • Sum of first 11 terms, \(S_{11}\) = a + (a + d) + (a + 2d) + (a + 3d) + …. + (a + 9d) + (a + 10d)
      o Implies, \(S_{11}\) = 11a + d * (1 + 2 + 3 + 4 + … + 9 + 10) = 11a + d * 10 * 11/2 = 11a + 55d
      o We are given, \(S_{11}\) = 275 = 11a + 55d
      o Thus, a + 5d = 25 ……… (1)

    • We can write the sum of first 12 terms as \(S_{12}\) = \(S_{11}\) + \(12^{th}\) term
      o Implies, \(S_{12}\) = 275 + (a + 11d) = 275 + (a + 5d) + 6d = 300 + 6d
      o We are given that d is a positive integer

    • Now, if we observe the answer choices, the only option which is in the form of 300 + 6d is 324

Therefore, \(S_{12}\) can be equal to 324

Hence the correct answer is Option D.

Answer: D

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Sum of first 11 terms of an arithmetic progression is 275.

Which of the following can be the sum of the first 12 terms, if the first term and the common difference are positive integers?

Arithmetic progression: -
First term = a
Commond difference = d
Number of terms = n
Sum of n terms = n/2{2a + (n-1)d}

Sum of 11 terms = 11/2 {2a + 10d} = 11a + 55d = 11 {a+5d} = 275
a + 5d = 25
2a + 10d = 50

12th term = a + 11d

Sum of first 12 terms = 12/2 {2a + 11d} = 6{2a + 11d} = 6(2a + 10d) + 6d = 300 + 6d

Only Option D 324 is of the form since d is a positive integer.

IMO D
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We get from the first statement that 11/2(2a+10d) = 275 => a+5d = 25
We are given that a,d are integers => if a=5,d=4 , if a=10,d=3 , if a=15, d=2 (a=0,d=0 cases are omitted and all other values can be omitted as they are all negative)
For sum of 12 , we need to find 6(2a+11d) => 6(50)+6d => 300+6d.
Possible options are 318,312,324.
From the options it is 324(D)
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