7+11+15+....+999 = 3+7+11+15+...+999 - 3 =
= (1+2)+(3+4)+(5+6) +(7+8)+...+(499 + 500) - 3 = (999:2 = 499 rest 1)
=1+2+3+4+...+500 - 3=
Acum folosim formula: [tex]1+2+3+4+...+n = \frac{n\cdot(n+1)}{2} [/tex] si avem:
[tex] =\frac{500\cdot501}{2}-3 = 250\cdot 501 -3 = 125250 - 3[/tex] = 125247
