Question

Without adding, find the sum
$1+3+5+7+9$
$1+3+5+7+9+11+13+15+17+19$
$1+3+5+7+9+11+13+15+17+19+21+23$

Answer 1

$1+3+5+7+9=$ Sum of first $5$ odd numbers $=5^2=25$.
Similarly,
$1+3+5+7+9+11+13+15+17+19=$ Sum of first $10$ odd numbers $={10}^2=100$.
$1+3+5+7+9+11+13+15+17+19+21+23=$ Sum of first $12$ odd numbers $={12}^2=144$.