MCQ practice · SSC General Mathematics Board questions · Chapter 13: Finite Series
What is the sum of 21 terms of the series 3 + 5 + 7 + ......?
Think it through first, then pick an option.
- a 43
- b 46
- c 483
- d 966
Answer and explanation
Correct answer: c 483
a = 3, d = 2, n = 21; S = (21/2){6 + 20 × 2} = (21/2) × 46 = 483.
Why the other options are wrong
- a 43: This is the 21st term, not the sum. The working is: a = 3, d = 2, n = 21; S = (21/2){6 + 20 × 2} = (21/2) × 46 = 483.
- b 46: This is first plus 21st term; it was not multiplied by n/2. The working is: a = 3, d = 2, n = 21; S = (21/2){6 + 20 × 2} = (21/2) × 46 = 483.
- d 966: The sum was doubled; the factor 1/2 was missed. The working is: a = 3, d = 2, n = 21; S = (21/2){6 + 20 × 2} = (21/2) × 46 = 483.
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