Question

Question 86 (ii)

The sum of first n natural numbers is given by $\frac{1}{2}{n}^2+\frac{1}{2}n$. Find

The sum of first 11 natural number

Answer 1

Given, sum of first n natural numbers $=\frac{1}{2}{n}^2+\frac{1}{2}n$
Sum of first 11 natural number
$=\frac{1}{2}(11{)}^2+\frac{1}{2}(11)=\frac{1}{2}\times 121+\frac{11}{2}$ [put n=11]
$=\frac{121}{2}+\frac{11}{2}=\frac{132}{2}=66$