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The sum of first 20 odd natural numbers is

A. 100 B. 210 C. 400 D. 420 Answer: Option C
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Solution(By Apex Team)

First 20 odd natural numbers are 1, 3, 5, 7, 9, 11, 13, 15, . . . . ., 39 Here a = 1, d = 2, n = 20 $\begin{aligned}&\therefore S_{20}=\frac{n}{2}[2a+(n-1)d]\\ &=\frac{20}{2}[2\times1+(20-1)\times2]\\ &=10(2+38)\\ &=10\times40\\&=400\end{aligned}$

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