1, MM: it is a unit of length and also a unit of rainfall, abbreviated as mm in English. ?

2. centimeter: it is a unit of length measurement, equal to one hundredth of one meter.

3. Kilometers: Kilometers, also called kilometers, are units of length and are usually used to measure the distance between two places.

4. Addition: one of the four basic operations, which refers to the calculation of combining two or more numbers and quantities into one number and quantity. The addition symbol is the plus sign (+).

5. The name "+"of each part of addition is a plus sign, the numbers before and after the plus sign are addends, and "=" is equal to the number and the number after it.

6. The essence of subtraction: subtracting a number is equal to adding the reciprocal of this number. ?

7. Check calculation: After calculating the problem, calculate it again by inverse operation (such as subtraction, addition, subtraction, multiplication and division, multiplication) to check whether the results of the previous operation are correct.

8. The function of checking calculation: checking calculation can effectively detect errors in the calculation process.

9. Quadrilateral: A closed three-dimensional figure surrounded by four line segments connected end to end on the same line is called quadrilateral.