This video shows that sum of all natural numbers can be represented as an infinite geometric series of all the powers of 9: 1+2+3+4+... = 1+9+81+729+... Then this formula for an infinite geometric series is applied: 1/(1-x) = 1+x+x^2+x^3+x^4+... and the result 1+2+3+4+...= -1/8 is demonstrated again.
