Question

Use​ Gauss's approach to find the following sums​ (do not use​ formulas). 1+3+5+7+...999

Answer 1

$S=1+3+5+\cdots+995+997+999$
$S=999+997+995+\cdots+5+3+1$
$\implies 2S=(1+999)+(3+997)+(5+995)+\cdots+(995+5)+(997+3)+(999+1)$

$2S=\underbrace{1000+1000+\cdots+1000+1000}_{500\text{ times}}$

(We are adding 500 instances of 1000 because that's how many odd numbers there are between 1 and 1000, inclusive.)

$2S=500\times1000=500000$
$\implies S=250000$