Junior high school mathematics problems (1 / 1998-1) * (1 / 1997-1) * (1 / 1996-1). (1 / 1001) * (1 / 1000-1) There are bonus points for the correct answers handed in tonight

Junior high school mathematics problems (1 / 1998-1) * (1 / 1997-1) * (1 / 1996-1). (1 / 1001) * (1 / 1000-1) There are bonus points for the correct answers handed in tonight

(1 / 1998-1) * (1 / 1997-1) * (1 / 1996-1). (1 / 1001-1) * (1 / 1000-1) = (- 1997 / 1998) * (- 1996 / 1997) * (- 1995 / 1996). * (- 1000 / 1001) * (- 999 / 1000) has a total of 999 items, so the final result is that the negative denominator is about = - 999 / 1998 = - 1 / 2

A clever calculation problem: multiply the difference of 1 / 1998 minus 1 by the difference of 1 / 1997 minus 1. Multiply the difference of 1 / 1000 minus 1 to calculate

【1/n─1】【1/（n-1）─1】.【1/（n-998）─1】
=【1-n/n】【（2-n）/（n-1）】.【（999-n）/（n-998）】
Extract minus sign about 999 items
=【n-1/n】【（n-2）/（n-1）】.【（n-999）/（n-998）】
=-【n-999/n】
When n = 1998
=-1/2

18 27 36 108 216 306 1098 these are multiples of 9. What is the law of the sum of the numbers in each digit of these numbers?
After research, what characteristics do you find in multiples of 9

The number of each digit is a multiple of 9, so the original number must be a multiple of 9

Fill 1, 2, 3, 4, 6, 9, 12, 18, 36 in the 3 * 3 box, so that the horizontal, vertical and oblique multiplication will get 216

2 36 3
9 6 4
12 1 18