3(1+2+3+4+5+...+1000)+(3+4+5+...+1000)×(-3) =
3(1+2+3+4+5+...+1000) - 3(3+4+5+...+1000) =
3[(1+2) + (3+4+5+...+1000)] - 3(3+4+5+...+1000) =
3(1+2) + 3(3+4+5+...+1000) - 3(3+4+5+...+1000) = 3(1 + 2) = 3 × 3 = 9
