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Class 10MathematicsArithmetic Progressions (AP)Finding the sum of the first $n$ terms of an APExpert
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The (9)th term of an arithmetic progression is (46) and the (21)th term is (106). What is the sum of the first (21) terms?

Answer and explanation

Correct answer: 1176

For an AP, \(T_n=a+(n-1)d\). Thus, \(a+8d=46\) and \(a+20d=106\). Subtracting gives \(12d=60\), so \(d=5\) and \(a=6\). Therefore, \(S_{21}=\frac{21}{2}[2a+20d]=\frac{21}{2}[12+100]=1176\). Option 1113 can result from using \(21\times53\), but the average of the first and 21st terms is \(56\), not 53. Exam tip: when two terms are given, form equations first to find \(a\) and \(d\).

Related tags

Arithmetic ProgressionSum Of ApNth TermLinear EquationsClass 10 MathematicsFinding The Sum Of The First $N$ Terms Of An ApFinding The Sum Of The First N Terms Of An ApArithmetic Progressions (Ap)
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10 more MCQs
  1. 1If an arithmetic progression has first term \(a\), common difference \(d\), and \(n\) terms, which formula correctly represents the sum of its first \(n\) terms?
  2. 2Find the sum of the first (15) terms of the AP (4,8,12,\ldots).
  3. 3Which formula correctly represents the sum of the first \(n\) terms of an AP with first term \(a\) and common difference \(d\)?
  4. 4What is the sum of the first (25) terms of the AP (1,3,5,\ldots)?
  5. 5What is the sum of the first (12) terms of the AP (10,20,30,\ldots)?
  6. 6What will be the sum of the first (10) terms of the AP (7,14,21,\ldots)?
  7. 7If the first term of an AP is (5), the last term is (45), and the number of terms is (9), what is the sum?
  8. 8Find the sum of the first (30) terms of the AP (3,6,9,\ldots).
  9. 9What is the sum of the first (8) terms of the AP (20,18,16,\ldots)?
  10. 10If the AP is (9,13,17,\ldots), what is the sum of the first (11) terms?
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