1, pen subtraction, first align the same number of digits, when the reduced number of digits is not enough from the single digit, retreat from the previous number 1 and make ten, and then reduce the number added to the standard.

2. Check the addition formula to exchange the positions of addends, or add and subtract one addend to see if it is equal to another addend.

3, a number multiplied by 1, the product is not necessarily greater than this number multiplied by 0.

4. Compare the sizes of numbers with the same number of digits; Firstly, compare the highest number of these two numbers; If the digits in the highest order are the same; Just compare the next one; And so on.

5. A closed figure with four straight edges and four corners. We call it a quadrilateral. A quadrilateral is characterized by four straight sides and four corners.