What is the sum of first 20 odd natural numbers?

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Solution

$\begin{matrix} T h e f i r s t 20 o d d n a t u r a l n u m b e r i s g i v e n b y 1, 3, 5, 7, 9, . . . . .37, 39. h e r e, a = 1 d = 2 a n d n = 20 N o w, S_{n} = \frac{n}{2} [2 a_{1} + (n - 1) d] S_{20} = \frac{20}{2} [2 (1) + (20 - 1) 2] = 10 [2 + (19) 2] = 10 [2 + 38] = 10 [400] = 400 H e n c e, t h e s u m o f f i r s t 20 o d d n a t u r a l n u m b e r i s 400. \end{matrix}$

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The sum of first 20 odd natural numbers is:

20

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