Question

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

Answer 1

The sum of first consecutive ${}^{\prime }{n}^{\prime }$ odd numbers, $=1+3+5+...+(2n-1)=n^2$
Here, $2n-1=19=>n=10$
Hence, $1+3+5+7+9+11+13+15+17+19={10}^2=100$