1, additive commutative law: Two numbers are added to exchange the position of addend, and the sum is unchanged.
2. Law of additive combination: When three numbers are added, the first two numbers are added first, or the last two numbers are added first, and then the third number is added, and the sum remains unchanged.
3. Multiplication and exchange law: when two numbers are multiplied, the position of the exchange factor remains unchanged.
4. Multiplication and association law: When three numbers are multiplied, the first two numbers are multiplied, or the last two numbers are multiplied first and then the third number, and their products are unchanged.
5. Multiplication and distribution law: When two numbers are multiplied by the same number, you can multiply the two addends by this number respectively, and then add the two products, and the result is unchanged, such as: (2+4) × 5 = 2× 5+4× 56.
The essence of division: in division, the dividend and divisor are expanded (or reduced) by the same multiple at the same time, and the quotient remains unchanged. O is divided by any number that is not O. Simple multiplication: the multiplicand and the end of the multiplier are multiplied by O. You can multiply 1 before o first, and zero does not participate in the operation, and add a few zeros at the end of the product.
Simple calculation method:
1. In the operation at the same level, the positions of numbers can be exchanged at will, but they should be exchanged with the previous symbols. (addition or multiplication commutative law)
2. In the operation at the same level, parentheses can be added or deleted directly after the plus sign or multiplication sign; Add brackets after minus sign and division sign, delete brackets, and change the symbols in brackets. (law of addition or multiplication)
3. Make up one method, make up ten methods, make up a hundred methods, and make up a thousand methods: "Make up nine before, make up ten after".
Be sure to remember: 25 replace 4 with 100, 125 replace 8 with 1000 (rounding idea).