Question

Without adding, find the sum:

1 +3 + 5+7 + 9 +11 +13+ 15 + 17+19

Answer 1

Here , there are ten odd numbers. (n=10)
we know that sum of first $n$ odd numbers = $n^2$

$\therefore$ 1 +3 + 5+7 + 9 +11 +13+ 15 + 17+19

$={10}^2=100$