One or four operations:

1, in the formula without brackets, if there is only addition and subtraction, it should be calculated from left to right.

2. In the formula without brackets, if there is only multiplication and division, it should be calculated from left to right.

3. There are multiplication, division, addition and subtraction in the formula without brackets. You should calculate multiplication and division first, and then add and subtract.

4. There are brackets in the formula. The parentheses are calculated first, and then calculated in the order of "multiply first, then divide, then add and subtract".

Second, the laws of addition and multiplication:

1, where the two addends exchange positions and the sum remains unchanged, is called additive commutative law. The letter means: A+B = B+A.

2. Add the first two numbers, or add the last two numbers first, and the sum is unchanged. This is the so-called law of addition and association.

Expressed in letters: (a+b)+ c =a+(b +c)

3. Two factors exchange positions and the product remains the same. This is called the multiplicative commutative law. The letter means a× b = b× a.

4. Multiply the first two numbers, or multiply the last two numbers, and the product remains unchanged. This is the so-called law of multiplication and association.

Use letters to represent (a × b)× c = a ×(b × c)

5. When the sum of two numbers is multiplied by a number, you can multiply it separately and then add it. This is the so-called law of multiplication and division.

Expressed in letters: (a+b) × c = a× c+b× c.

Third, the meaning and nature of decimals:

1. Divide 1 meter into 10 parts, each part is (1 decimeter), that is, (110) meter, that is, (0. 1 meter.

2. Divide 1 m into 100 parts, each part is (1 cm), that is, (1100) m, that is, (0.0 1 m).

3. Divide 1m into 1000 parts, each part is (1mm), that is, (11000) meters, that is, (0.00 1m).

4. Fractions with denominators of 10, 100, 1000 ... can be expressed in decimals.