Answer
a. $\frac{3\times4}{2} = 6$
b. $\frac{5\times6}{2} = 10$
c. $\frac{9\times10}{2} = 45$
Work Step by Step
a. $n=3$
$1+2+3=\frac{3\times4}{2} = 6$
b. $n=5$
$1+2+...+5=\frac{5\times6}{2} = 10$
c. $n=9$
$1+2+...+9=\frac{9\times10}{2} = 45$
The formula $\frac{n\times(n+1)}{2}$ works correctly on odd numbers too.
