1、(1)一半 半径 底 高 圆周长的一半 半径 πr²
(2)2 dm 12.56 dm 12.56 dm²
(3)相等
(4)20 100
(5)9
2、(1)×
(2)×
(3)×
(4)√
3、(1)3²× 3.14 = 28.26 cm²
(2)(8 ÷ 2)² × 3.14 = 50.24 dm²
(3)(18.84 ÷ 3.14 ÷ 2)² × 3.14 = 28.26 m²
4、(1)10 × 10 - (10 ÷ 2)² × 3.14 = 21.5 dm²
(2)(12 ÷ 2)² × 3.14 ÷ 2 - (12 ÷ 2 ÷ 2)² × 3.14 = 28.26 cm²
5、(1)5² × 3.14 = 78.5 m²
(2)(12 ÷ 2)² × 3.14 = 113.04 cm²
(3)(25.12 ÷ 3.14 ÷ 2)² × 3.14 = 50.24 m²
(4)设长方形的宽为x cm,圆的半径也为x cm
6.28x = 3.14x²
解得x = 2 cm
圆的面积为2² × 3.14 = 12.56 cm²