When calculating rectangles and squares, just like calculating line segments, it is necessary to calculate not only small rectangles and squares composed of a basic unit, but also several small rectangles and squares. We should count according to a certain order and law, so that we won't count too much or too little.

Pay special attention to the characteristics of a square when counting, and don't forget that a square must have four sides. So two or three small squares can't spell a big square. Only four small squares and nine small squares can spell a big square.

Data expansion:

Knowledge points of rectangle and square in mathematics of grade three;

1. A closed figure with four straight sides and four corners is called a quadrilateral. A quadrilateral is characterized by four straight sides and four corners. The characteristic of rectangle is that it has two lengths and two widths. All four corners are right angles and the opposite sides are equal. A square is characterized by four right angles and four equal sides.

2. Rectangular and square are special parallelograms. Parallelogram is characterized by equal opposite sides and diagonal lines. Parallelogram is easy to deform. The length of a closed figure is its perimeter. Make a big rectangle with several squares. The closer its length and width are, the shorter its circumference is. The greater the difference in length and width, the longer the circumference.

3. The summary method of perimeter measurement is ruler measurement (regular figure), in which the length of each side is measured with a ruler, and the sum of all side lengths is the length of one week of the figure; Rope measurement method (irregular figure), use a rope (or line) to circle the edge of the figure, cut off the redundant part, and then straighten it, and measure its length as the circumference of the figure.

4. The formula is: the circumference of a rectangle = (length+width) ×2 or length ×2+ width × 2; Length of rectangle = perimeter ÷2- width; Width of rectangle = perimeter ÷2- length; The circumference of a square = side length × 4; Side length of a square = perimeter ÷4.