Question

Choose the correct answer in each of the following questions:

The sum of first 20 odd natural numbers is [CBSE 2012]

(a) 100 (b) 210 (c) 400 (d) 420

Answer 1

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

$$\begin{gathered} =\frac{20}{2}\left(1+39\right)\left[S_n=\frac{n}{2}\left(a+l\right)\right] \\ =10\times 40 \\ =400 \end{gathered}$$

Hence, the correct answer is option C.