Multiple choice

How many terms of the series 1 + 3 + 5 + …………. amount to 123454321.

  1. 11011

  2. 11101

  3. 1111

  4. 11111

Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

Series 1 + 3 + 5 + ... is sum of first n odd numbers. Formula for sum of first n odd numbers = n^2. Setting n^2 = 123454321, we get n = sqrt(123454321) = 11111 (since 11111^2 = 123454321). Therefore, 11111 terms are needed.