Question
The sum of first 20 odd natural numbers is
Answer: Option C
First 20 odd natural numbers are 1, 3, 5, 7, 9, 11, 13, 15, . . . . ., 39
Here a = 1, d = 2, n = 20
$$\eqalign{
& \therefore {S_{20}} = \frac{n}{2}\left[ {2a + \left( {n - 1} \right)d} \right] \cr
& = \frac{{20}}{2}\left[ {2 \times 1 + \left( {20 - 1} \right) \times 2} \right] \cr
& = 10\left( {2 + 38} \right) \cr
& = 10 \times 40 \cr
& = 400 \cr} $$
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First 20 odd natural numbers are 1, 3, 5, 7, 9, 11, 13, 15, . . . . ., 39
Here a = 1, d = 2, n = 20
$$\eqalign{
& \therefore {S_{20}} = \frac{n}{2}\left[ {2a + \left( {n - 1} \right)d} \right] \cr
& = \frac{{20}}{2}\left[ {2 \times 1 + \left( {20 - 1} \right) \times 2} \right] \cr
& = 10\left( {2 + 38} \right) \cr
& = 10 \times 40 \cr
& = 400 \cr} $$
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