Choose the correct answer in each of the following questions:
Question:

Choose the correct answer in each of the following questions:

The sum of first 20 odd natural numbers is      

(a) 100

(b) 210

(c) 400

(d) 420

Solution:

The first 20 odd natural numbers are 1, 3, 5, …, 39.

These numbers are in AP.

Here, a = 1, l = 39 and n = 20

∴ Sum of first 20 odd natural numbers

$=\frac{20}{2}(1+39) \quad\left[S_{n}=\frac{n}{2}(a+l)\right]$

$=10 \times 40$

$=400$

Hence, the correct answer is option C.

 

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