Sum of all natural numbers from 1 to 1999 Not (1 + 1999) x1999 / 2 It's 1 + 2 + 3 + 4 + 5 + 6 + 7 + 8 + 9 + 1 + 0 + 1 + 1 + 1 + 2 + 1 + 3 + 1 + 4 + 1 + 5 + 1 + 6 + 1 + 7. + 1 + 9 + 9 + 9

Sum of all natural numbers from 1 to 1999 Not (1 + 1999) x1999 / 2 It's 1 + 2 + 3 + 4 + 5 + 6 + 7 + 8 + 9 + 1 + 0 + 1 + 1 + 1 + 2 + 1 + 3 + 1 + 4 + 1 + 5 + 1 + 6 + 1 + 7. + 1 + 9 + 9 + 9

It shouldn't be the sum of natural numbers. It should be the sum of all numbers
The original formula = (0 + 1 + 2 + 3 + 4 + 5 + 6 + 7 + 8 + 9) + (0 + 1 + 2 + 3 + 4 + 5 + 6 + 7 + 8 + 9 + 10 × 1) + (0 + 1 + 2 + 3 + 4 + 5 + 6 + 7 + 8 + 9 + 10 × 2) + (0 + 1 + 2 + 3 + 4 + 5 + 6 + 7 + 8 + 9 + 10 × 3) + +(0+1+2+3+4+5+6+7+8+9+10×199)= (0+1+2+3+4+5+6+7+8+9) ×200+(1+2+3+4…… +199) ×10=45×200+19900×10=9000+199000=208000

The two natural numbers adjacent to 100 are () and (), the two even numbers adjacent to 100 are () and (), and the two odd numbers adjacent to 99 are ()
And ()

The two natural numbers adjacent to 100 are (99) and (101), the two even numbers adjacent to 100 are (98) and (102), and the two odd numbers adjacent to 99 are (97) and (101)