This kind of problem is named "arithmetic progression", and the formula for solving this kind of problem is

(first item+last item) × number of items ÷2

In this problem, the first item is "1", the last item is "100", and the number of items is the total number in the index column. This problem has 100 items, so the number of items is 100.

Then the original formula = (1+100) ×100 ÷ 2.

= 10 1× 100÷2

= 10 100÷2、

=5050