Răspuns :
1-2+3-4+...+2009-2010=-1-1-1...-1 (de la 2*1=2 la 2*1005=2010) dec i 1005 de -1
total -1005
atunci
-1005+2011=1006
1 + 3 + ... + 2009 + 2011
-2 - 4 - ... - 2010 ⇔ 1005 termeni !
-------------------------------
-1 - 1 - ... - 1 + 2011
↑_____________↑
1005 * ( - 1) + 2011 = +1006
-2 - 4 - ... - 2010 ⇔ 1005 termeni !
-------------------------------
-1 - 1 - ... - 1 + 2011
↑_____________↑
1005 * ( - 1) + 2011 = +1006