1, additive commutative law: In the addition operation of two numbers, according to the calculation order from left to right, the two addends are added, the positions of the addends are exchanged, and the sum is unchanged. The letter a+b=b+a a+c=c+a, and the number1+2 = 2+116+30 = 30+16.
2, the law of additive combination: the law of additive combination is the addition of three numbers, first add the first two numbers, or add the last two numbers first. And invariance, which is called additive associative law. The letter means a+b+c=a+(b+c), and the number means18+5+15 =18+(5+15) = 38.
3. Multiplicative commutative law: Multiplicative commutative law is the arithmetic of multiplication. When two factors are multiplied, the position of the exchange factor and the product remains unchanged, which is called the multiplication exchange law. Multiply the majority, and the product of any two numbers remains the same.
4. The law of multiplication and association: it is a simple law, which is involved in the second volume of the fourth grade mathematics textbook of the People's Education Press: in the multiplication operation of two numbers, according to the calculation order from left to right, the two factors are multiplied, and the position of the exchange factor remains unchanged. Specifically, when two numbers are multiplied, the positions of exchange factors and their products remain unchanged. It's called multiplicative commutative law.
Calculation method:
The common methods of using pencil and paper multiplier need a small number (usually any two numbers between 0 and 9) to store or query the product multiplication table, but a farmer multiplication algorithm does not need it.
Multiplying a number with multiple decimal places is troublesome and error-prone. The universal logarithm was invented to simplify this calculation. The slide rule allows numbers to be quickly multiplied by about three digits of precision. Since the early 20th century, mechanical calculators, such as Marchant, have automatically multiplied as many as 10 digits. Modern electronic computers and calculators greatly reduce the need for manual multiplication.