10+20+30+...+800= (dai factor comun 10)
=10(1+2+3+...+80)
=10(80x81):2
=10x6480:2
=64800:2=32400
10+20+30+...800=10(1+2+3+...80) suma din paranteza o calculam cu formula Gauss, 1+2+3+....+80=80*81/2=6480/2=3240, deci suma care ne intereseaza va fi
=10*3240=32400