How many terms of the series 1+3+5+7+\dots amount to a sum equal to 12345678987654321?
- A. 11111111
- B. 110000011
- C. 111101111
- D. 111111111 ✓
Correct Answer: D. 111111111
Explanation
The sum of the first n odd natural numbers is n^2. Thus, n^2 = 12345678987654321. Taking the square root of this palindromic sequence of digits gives n = 111111111 (nine ones).
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