Ca sa afli suma nuemerelor consecutive folosesti formula lui Gauss: n(n+1)/2.
1+2+3+....+2004 = 2004*(2004+1)/2=2004*2005=(1+2004)*2004
2005+2*(1+2+3+....+2004)=(1+2004)+(1+2004)*2*2004=(1+2004)*(1+2*2004)=(1+2004)*(1+2004+2004)=(1+2004)*(1+2004)=(1+2004)²