1, rectangle: perimeter = (length+width) ×2—— (length = perimeter ÷2- width; Width = perimeter ÷2- length). Letter representation: C=(a+b)×2. Area = length × width. The letter means: S=ab.

2. Square: perimeter = side length ×4. Letter: C=4a. Area = side length × side length. The letter means: S=a2.

3. The area of parallelogram = base × height. Letters indicate:? S = ah.

4. Area of triangle = base × height ÷2 —— (base = area × 2 height; Height = area × 2 bottom). The letter means: s = ah÷2.

5. Trapezoidal area = (upper bottom+lower bottom) × height ÷2. Letters indicate:? S =(a+b)h \2. Upper bottom = area× 2÷ height-lower bottom, and lower bottom = area× 2÷ height-upper bottom; Height = area ×2 (upper bottom+lower bottom).

6. Derivation of parallelogram area formula: cutting, translation and cutting.

7. Derivation of triangle area formula: rotating splicing method. Parallelogram can be changed into rectangle; Two identical triangles can be combined into a parallelogram, and the length of the rectangle is equivalent to the base of the parallelogram; The base of parallelogram is equivalent to the base of triangle; The width of the rectangle is equivalent to the height of the parallelogram; The height of a parallelogram is equivalent to the height of a triangle.

The area of a rectangle is equal to the area of a parallelogram, which is equal to twice the area of a triangle. Because the area of rectangle = length × width, and the area of parallelogram = bottom × height. Because parallelogram area = base × height, triangle area = base × height ÷2.

8. Derivation of trapezoidal area formula: rotating splicing method.

9. Two identical trapezoids can be combined into a parallelogram; The base of parallelogram is equivalent to the sum of the upper and lower bases of trapezoid; The height of parallelogram is equivalent to the height of trapezoid; The area of parallelogram is equal to twice the area of trapezoid, because the area of parallelogram is equal to bottom × height, so the area of trapezoid is equal to (upper bottom+lower bottom) × height ÷2.

10, parallelogram with equal base and equal height has the same area; Triangles with equal bases and equal heights have equal areas; The area of a parallelogram with equal base and equal height is twice that of a triangle.

1 1. The rectangular frame is drawn as a parallelogram with the same perimeter and smaller area.

12. Combined figure area (or shadow area): convert it into a simple figure that has been learned and calculate it through addition and subtraction.

Calculation method of division with divisor as decimal.

1. First expand the divisor and dividend by the same multiple, so that the divisor becomes an integer, and then calculate according to the law that the divisor is an integer. Note: If there are not enough digits in the dividend, make up the dividend with 0 at the end.

2. In practical application, the quotient obtained by fractional division can also be rounded to a certain number of decimal places as needed to obtain the approximate number of quotients.