Question

Without adding , find the sum. Explain answer (1+3+5+7+9+11+13+15+17)

Answer 1

First Term = a = 1 
Last  Term = l = 17 
Total  Terms = 9 



$S_{n} = \frac{n}{2} [ a + l }$

$S_{9} = \frac{9}{2} [ 1 + 17 ]$

$S_{n} = \frac{9}{2} [ 18 ]$

                  = 9 (9)  
                  = 81 


Sum = 81

Answer 2

Hy friend, here's ur ans....
(1+3+5+7+9+11+13+15+17)
As u know, there are 9 terms (9 numbers)
And the formula to find the sum of odd numbers is (n)²

So (n)²
(9)²
81

ANS. 81

U can verify the answer by actual addition....