Question

Without adding, find the sum: $1+3+5+7+9$

Answer 1

Find the sum of $1+3+5+7+9$.

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

It can be observed that here sum of first 5 odd natural number is to be calculated.

Thus,

$n=5$

The sum of the first $5$ odd natural numbers is,

$\begin{array}{rcl}1+3+5+7+9& =& 5^2\\ & =& 5\times 5\\ & =& 25\end{array}$

Thus, the sum is $25$.