Introduction to Special Series,
Sum of first n natural numbers
1+2+3+…+n=\frac{n(n+1)}{2}

Inspirational Story of Mathematician Gauss when he was in elementary school told by Prof. Arvind Gupta

Sum of Squares of first n natural numbers
1^2+2^2+3^2+…+n^2=\frac{n(n+1)(2n+1)}{6}

Sum of Cubes of first n natural numbers
1^3+2^3+3^3+…+n^3= \left ( \frac{n(n+1)}{2} \right )^2