Question

Find the sum of first 100 natural numbers.

Or

$\sum _{x=1}^{100}x=?$

Answer 1

${\sum }_{x=1}^{100}x=1+2+3+...+100$
Clearly, it is an AP with $a=1,d=1,n=100$
Sum of first $n$ natural numbers $=\frac{n(n+1)}{2}$
So, sum from $1$ to $100=\frac{100\times 101}{2}=5050$