Question

Without adding, find the sum. $\left(\mathrm{i}\right)1+3+5+7+9$

Answer 1

As sum of first $n$ natural odd numbers is $n^2$.

In $1+3+5+7+9$ there are $5$ consecutive odd numbers.

So,

$$\begin{gathered} \therefore \mathrm{sum}=5^2 \\ =25 \end{gathered}$$

Hence, the sum of $1+3+5+7+9$ is $25$.