In the third grade of primary school, I began to learn the concept of area. Area generally refers to the size of the area occupied by a plane figure, which is used to describe the "size" of an object on the plane. In the third grade of primary school, we mainly studied the formulas for calculating the area of polygons and rectangles.

A, rectangular area calculation formula:

A rectangle is a plane figure with four sides, each pair of opposite sides is equal, and all angles are right angles. The formula for calculating the area of a rectangle is: the area of the rectangle = the length of the rectangle × the width of the rectangle.

In the formula, the length of the rectangle refers to the long side of the rectangle, and the width of the rectangle is the short side of the rectangle. You can measure the length and width of a rectangle with a ruler, ruler and other tools, and then bring it into the formula for calculation.

Second, the square area calculation formula:

A square is a special rectangle. All sides are equal and all angles are right angles. The formula for calculating the square area is: the area of the square = the square of the side length of the square. This formula is similar to the formula for calculating the area of a rectangle, except that the sides of a square are equal.

Three, polygon area calculation formula:

Polygon is a closed plane figure composed of several line segments. For example, triangles and trapezoids are common polygons. To calculate the area of a polygon, we can divide it into some small triangles and trapezoid, and then calculate it according to the basic formula.

1, the area calculation formula of triangle. A triangle is a closed plane figure composed of three line segments, and the formula for calculating the area of the triangle is: the area of the triangle = bottom × height ÷2.

Where the base is the base of the triangle and the height is the height perpendicular to the base. Because a triangle can be divided into a rectangle or two isosceles triangles, this formula can also be regarded as a special case of the formula for calculating the area of rectangles and isosceles triangles.

2. Calculation formula of trapezoidal area. A trapezoid is a plane figure with two parallel bases and four sides with the same slope. The formula for calculating trapezoidal area is: trapezoidal area = (upper bottom+lower bottom) × height ÷2. Wherein, the upper bottom edge and the lower bottom edge are the lengths of the upper bottom edge and the lower bottom edge of the trapezoid respectively, and the height is the vertical distance between the two bottom edges.

To sum up, the formulas for calculating the area of the third grade of primary school are rectangle, square, triangle and trapezoid. By learning these formulas, students can understand the calculation methods of various plane graphic areas and use different formulas to calculate them, which is helpful to further understand the basic concepts and methods of mathematics.