Every number is "9". The formula for quick calculation is: add 1 to each head, multiply it by 10, subtract the added number, and finally put 1. All ten digits are 9. The formula for quick calculation is: 100 minus the previous number and then minus the latter number. 100 MINUS everyone, the results are multiplied by each other, accounting for 2 places. The heads are the same and the tails are complementary (tail complementarity: the mantissas add up to 10). The formula for quick calculation is: head times head 1, and tail times tail 2 digits.
When the two digits are added together, the single digits just add up to a whole ten, and the decimal number is rounded directly. When two digits are added, the single digits can't make up a whole ten, but when they are split, they make up a whole ten. When two digits are added together, the single digits can't make up a whole ten, and the last few digits can also be split.
Two digits are subtracted, and the single digits are the same, which is directly reduced to 0. Two-digit subtraction, subtraction is greater than the minuend single digit, direct subtraction. When two numbers are subtracted, the minuend is smaller than the minuend, and it is meined into the same number as the minuend.
Two simple multiplications and one addition. The specific method is: the multiplier of more than ten is expressed by letters and mathematical methods 10+a, where A is a single digit, and the other multiplier is expressed by B, which is also a single digit.
Then the formula is (10+a)*b, and this algebraic formula is 10*b+a*b, so the formula is an integer 10 multiplied by two one-digit numbers, and the result can be obtained by oral calculation.