2.
The sum
12!+23!+34!+
+20222023! Can be expressed as
a−1b!, where
a and
b are positive integers. What is
a−b？
[提示]
k(k+1)!=k+1(k+1)!−1(k+1)!=1k!−1(k+1)!
我的教甄準備之路 裂項相消，
https://math.pro/db/viewthread.php?tid=661&page=2#pid1678
6.
Among the 900 three-digit numbers from 100 to 999, how many of them the sum of the three digits is smaller than 15？
