When n

However, starting from 12, due to the absolute value, the sequence starts to increase from 0, and at this time (n+>; = 12), the formula of the sum of the first n items is the negative number of the previous formula, because it has changed from negative growth to positive growth. If you understand it here, you will get half.

Since it starts from the item 12, the value of the first item 1 1 should be added, so it is -Sn+s 1, but from the item 12 to the item n=23, that is, the order is from 0, 1 2.

So the sum of the first n items before 23 is equal to+11->; 0, because it became a negative sign, 0-& gt； 0->; 1 1. So we need+two s 1 1.

I don't know if you can understand ... If not, you can keep asking.