-3+1+5+9+..+393+397 =
= -3 + (4×1-3 + 4×2-3 + 4×3-3 +...+ 4×99-3 + 4×100-3) =
= - 3 + [4 × (1+2+3+...+99+100) - (3 + 3 +...+3) de100ori ] =
= - 3 + (4 × 100 × 101 : 2 - 3 × 100) =
= -3 + (200 × 101 - 300) =
= - 3 + (20 200 - 300) =
= - 3 + 19 900 =
= 19 900 - 3 =
= 19 897
