1. Jia: 19 19 Good friend, 2828 holding hands, 3737 really close, 4646 walking together. Make a pair of hands in half. One plus nine, ten tadpoles, two plus eight, ten old ducks, three plus seven, ten old hens, four plus six, ten golden monkeys, five plus five, and ten big tigers.

2. subtraction: think of 1 when you see 9, 2 when you see 8, 3 when you see 7, and 4 when you see 6. When you see the big corner decimal system, switch the two numbers first.

First, the concepts of addition and subtraction.

1. addition: The operation of combining two numbers into one number is called addition.

A+b=c, A is called addend, B is also called addend, and A+B = C is called sum.

2. subtraction: the operation of finding another addend by knowing the sum of two numbers and one of them is called subtraction.

A-b=c, a is called minuend, b is called subtraction, and A-B = C is called difference.

0 can do addend and subtraction, but not minuend (primary school).

Second, the nature, laws and rules of addition and subtraction:

The size law of 1. sum: any number plus 0 equals the original number; The sum of two numbers that are not 0 must be greater than one of the addends.

2. The size of the difference: any number MINUS 0, the difference is equal to the original number; When two numbers that are not 0 are subtracted, the difference must be less than the minuend; When two equal numbers are subtracted, the difference must be equal to 0.

3. The variation law of the difference caused by meiosis and meiosis changes: two numbers are added, one addend adds a number, and the other addend subtracts this number, and the sum remains unchanged; Add two numbers, one addend is unchanged, the other addend adds a number, and the sum also adds this number; When two numbers are added, one addend remains the same, the other addend subtracts a number, and the sum also subtracts this number.

4. The change law of sum caused by the change of addend: two numbers are subtracted, and the minuend and the minuend add or subtract one number at the same time, and the difference is unchanged; Subtract two numbers, add a number to the minuend, and add this number to the difference;

Subtract two numbers, subtract one number from the minuend, and subtract this number from the difference if the minuend is unchanged; When two numbers are subtracted, the minuend remains the same, a number is added to the minuend, and this number is subtracted from the difference; When two numbers are subtracted, the minuend remains the same, and the minuend subtracts a number, which is added to the difference;

5. The algorithm of addition

(1) additive commutative law: Two numbers are added, the addend positions are exchanged, and the sum is unchanged.

a+b=b+a .

(2) The law of addition and association: when three numbers are added, the first two numbers are added and then added with the third number, or the last two numbers are added and then added with the first number, and the sum remains unchanged.

a+b+c=(a+b)+c=a+(b+c).

6. The operational nature of subtraction

(1) A number minus several numbers equals the sum of a number minus several numbers.

a-b-c=a-(b+c).

(3) A number minus the difference between two numbers equals a number minus the minuend in the difference, plus the minuend in the difference.

A-(b-c) = a-b+C. This attribute is also called a rule to delete or add brackets.

7. The operational nature of addition and subtraction mixed operation

(1) In the mixed operation of addition and subtraction without brackets, the result remains unchanged by changing the operation order.

a+b-c=a-c+b，a-b-c=a-c-b .

(2) Adding a number to the difference between two numbers is equivalent to adding a number to the minuend in the difference and then subtracting the minuend in the difference.

A+(b-c) = a+b-C. Also known as the rule of deleting and adding brackets.

8. Simple calculation of integer ten, integer hundred and integer thousand.

In addition and subtraction, there is a simple calculation (also called "fast algorithm"), in which the number is close to integer ten, integer hundred and integer thousand: two numbers are added (or subtracted), and one number is close to integer ten, integer hundred and integer thousand, which can be regarded as the sum (or difference) of integer ten, integer hundred, integer thousand and a number, and then added (or subtracted).