CA Foundation · Quantitative Aptitude · Sequence and Series
What is the value of Σ (2r + 3) for r = 1 to 20?
The sum equals 480. Splitting the sigma gives 2 times the sum of the first 20 natural numbers (2 × 210 = 420) plus 3 added 20 times (60), totalling 480. It also checks as an AP with first term 5 and last term 43.
- A460Correct
- B440
- C520
- D400
Explanation
Σ(2r + 3) = 2Σr + 3×20. Σr for 20 terms = 20×21/2 = 210, so 2×210 = 420. Adding 3×20 = 60 gives 480. Check as an AP: first term 5, last term 43, sum = 20×(5+43)/2 = 480. So the answer is 480.
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