Question

# Find the sum of the first 20 natural numbers. Choose the correct option:A. 210B. 220C. 230D. 240

Hint: It is known that the natural number starts from 1. Also, the sum of the first n natural number is given by: $\dfrac{n(n+1)}{2}$.

Now we know that the sum of the first n natural number is given by the formula $\dfrac{n(n+1)}{2}$.
$\Rightarrow n=20$
\begin{align} & \Rightarrow \dfrac{n(n+1)}{2} \\ & \Rightarrow \dfrac{20(20+1)}{2} \\ & \Rightarrow 10(21) \\ & \Rightarrow 210 \\ \end{align}
Note: It is important to keep in mind that zero is not the natural number. Also, the smallest natural number is one. This problem can also be solved by using the sum of n terms of an arithmetic progression formula i.e. $S_n = \dfrac{n}{2} [2a+(n-1)d]$ where first term (a) = 1, common difference (d) =1 and n =20.