Use the formula S =(n + 1)2 to find the sum of 1 + 2 + 3 + ... + 780.

1 + 2 + 3 + ... + 780 =

[ 780 x 781] / 2 =304,590

Guests answer is correct but your formula is wrong

This is what it should be.

S=n(n+1) / 2