Find the greatest prime divisor of the sum of the arithmetic sequence
1+2+3+.......+80
Sum up the sequence first using: n * (n+1) / 2 formula:[80 * 81] / 2 =3,240
Then factor 3240 = 2^3 * 3^4 * 5
As you can see, the largest Prime factor = 5
+81 
