3d
(2+4+6+....+2012) - (4+8+12+...+2012) -503 * 1005
la prima paranteza dam factor comun 2 iar la a doua dam factor comun 4
2(1+2+3+....+1006) - 4(1+2+3+...+503) -503 * 1005
aplicam suma lui Gauss pentru ambele paranteze [n(n+1)]:2, in care n=ultimul termen din sir
2*(1006*1007):2 - 4*(503*504):2 -503 * 1005
1006*1007 - 2*503*504 -503 * 1005
2*503*1007 - 2*503*504 -503 * 1005 impartim prin 503
2*1007 - 2*504 - 1005=
2014-1008 -1005=1
