Knowledge points:

The first set of formulas: the number of digits of the product is the sum of digits of two factors-1, the highest and lowest digits of the product are 1, the middle digit is the number of digits of the factor, and the digits on both sides are the same, and then1is reduced. (This is the number of palindromes)

The second set of formulas: the product consists of 1, 4, 2, 8, 5, 7, and the sequence is unchanged. You only need to determine the last digit to calculate the product (it would be better if you could directly calculate the first digit).

The third set of formulas: the unit of product is 1, and the first place is 9; The number of digits of the product is exactly the sum of the number of digits of two factors; Each bit of the product consists of 9, 8, 0 and 1. Just add 9 to the first digit and 0 to the penultimate digit. There is only one 8 and one 1.

The fourth set of formulas: from the ten numbers from 0 to 9, four numbers are randomly selected to form the largest four-digit number and the smallest four-digit number that are not repeated. Then subtract these two numbers, The four digits of the result are copied to form the subtraction of the largest four digits and the smallest four digits again. ........................................................................................................................ .....................................................

Summary: This paper introduces the fourth grade mathematics knowledge points: interesting formulas, and mathematics learning is also very interesting. I believe everyone can learn math well.