1, rational number: a number consisting of integers and fractions.

2. Both sides of the equation are multiplied or divided by the same number at the same time, and the divisor cannot be 0, and the result is still an equation.

3. Rectangular area formula: S=ab, where A is the length, B is the width and S is the area.

4. Number axis: Numbers are represented by points on a straight line, which is called number axis.

5. The law of addition and association: (a+b)+c=a+(b+c) three numbers are added, the first two numbers are added first, or the last two numbers are added first, and the sum is unchanged.