Classification discussion divides closed graphics into rectangles, triangles and circles. The biggest reason is that the area of a circle should be.

40 cm is the circumference of the figure, according to this calculation.

There is a mathematical principle that the length+width of a rectangle = 20cm. The smaller the difference between the two multipliers, the greater the product. Therefore, the maximum area surrounded by a rectangle is 100 square centimeter, that is, the difference between length and width is 0. The formula is (1/4C)?

The average side length of a triangle is about 14 cm, and the sum of two sides of the triangle is greater than the third side, so it is regarded as the largest right-angled triangle area, and the more it is about 70 square cm, the formula is

1/2( 1/3C)？ (approximately equal to it)

The diameter of a circle is 40÷π≈ 13 and the radius is about 6 cm.

πr？ ≈ 1 10 square centimeter is π[ 1/2(π/C)]?

The result speaks for itself.

For reference only, I hope it will help you.