(50÷2)? ×3. 14= 1962.5 cm2

02. Saw the largest circle from a rectangular board with a length of 6 decimeters and a width of 4 decimeters. How many square decimetres are the remaining boards?

6×4-(4÷2)? × 3.14 =11.44 square decimeter.

03. The circumference of the circular pool is 25. 12m. How many square meters is this swimming pool?

(25. 12÷3. 14÷2)? ×3. 14=50.24 square meters

04. A square iron sheet with a side length of 10 decimeter. Make the biggest circle. What is the area of this circle? What is the ratio of the area of a square to the area of this circle?

Area of the circle: (10÷2)? ×3. 14=78.5 square decimeter

Area of the square: 10? = 100 square decimeter

Proportion: 100:78.5=200: 157.