1, subtraction calculation
According to the fact that subtraction is the inverse operation of addition, one of its addends is subtracted from its sum. If the calculation is correct, this difference must be equal to another addend.
2. Addition calculation
According to additive commutative law, after exchanging the positions of addends, they are added. If the results of the two calculations are the same, the original calculation is correct.
3, abandon nine method checking calculation
Abandoning nine is also called more than nine methods. It is a checking method to check whether the four operations of addition, subtraction, multiplication and division are correct according to the characteristics of nine remainder.
Operating rules:
1, additive commutative law
In the addition of two numbers, the positions of the two addends are interchanged and the sum is unchanged. The letter means: a+b = b+a.
2. Additive associative law
Add three numbers, first add the first two numbers and then add another addend; Or add the last two numbers first, then add another addend, and the sum remains the same. The letter indicates: (a+b)+c = a+(b+c).
3. Multiplicative commutative law
In the multiplication operation of multiplying two numbers, the positions of the two multipliers are interchanged and the product remains unchanged. The letter means: a× b = b× a.
4. Multiplicative associative law
Multiply three numbers, first multiply the first two numbers, or multiply the last two numbers first, and the product remains the same. The letter indicates: (a× b )× c = a× (b× c).
5. Law of Multiplication and Distribution
Adding (or subtracting) two numbers and then multiplying by another number is equivalent to multiplying this number by two addends (subtracting) and then adding (subtracting) the two products to get the same number. Letter representation: ① (a+b) × c = a× c+b× c; a×c+b×c =(a+b)×c； ②a×(b-c)= a×b-a×c； a×b-a×c=a×(b-c).
6. Law of continuous decline
Two numbers subtract a number continuously, which is equal to the sum of the two numbers after subtracting this number, and the number remains unchanged; Letters: a-b-c = a-(b+c); A-(B+C) = A-B-C.
In the addition and subtraction of three numbers, the positions of the two numbers remain unchanged after the exchange. Letters: a-b-c = a-c-b; a-b+c=a+c-b .
7. The law of continuous segmentation
The continuous division of a number by two numbers is equal to the product of the two numbers after division, and the numbers are unchanged. The letter indicates: a ÷ b ÷ c = a ÷ (b× c); a \(b×c)= a \b \c .
In the multiplication and division of three numbers, the positions of the two numbers remain unchanged after the exchange. The letter means: a \b \c = a \c \b； b; a \b×c = a×c \b .