The carry addition and subtraction table within 20 is: "add nine MINUS one, add eight MINUS two, add seven MINUS three, add six MINUS four, add five MINUS five." How to use the formula? Take "plus nine MINUS one" as an example. "Add nine and subtract one" refers to the addition of a number and 9, and the number minus 1 is the unit of their sum.

The abdication subtraction within 20 is opposite to the carry addition within 20, that is, the abdication subtraction within 20 is converted into the addition within 10. The formula is: "minus nine plus one, minus eight plus two, minus seven plus three, minus six plus four, minus five plus five." Take "minus nine plus one" as an example. "Minus nine plus one" refers to a number MINUS nine, and the result of adding 1 to the unit of this number is their difference.

For example: 17-2 = () Distinguish which number is one and which number is ten. First, see if you can subtract one digit. If 7-2 is enough, please subtract from 10. Remember 7 and subtract 6. 5, get 5, and then the tenth 1 remains unchanged, and get 15.

The method of "minus eight plus two, minus seven plus three, minus six plus four, minus five plus five" is the same as "minus nine plus one".

Carry addition is a mathematical operation and addition. When two numbers are added, if the result of one bit is greater than or equal to 10, you need to count 1 to the next bit, which is the carry, and its formula is the carry addition formula.