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The sum 1 + 3 + 5 + 7 + 9 + 11 + 13 + 15 + 17 equals:
A64
B72
C81
D100
Answer & Solution
Correct answer: C. 81
The sum of the first n odd natural numbers is n². Here we have the first 9 odd numbers (1 through 17), so the sum is 9² = 81. This is also the basis of the 'repeated subtraction' method for finding √81.
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