extension Double { mutating func square() { let pi = 3.1415 self = pi * self * self } } var Trial1 = 3.3 Trial1.square() print("圆的面积为: \(Trial1)") var Trial2 = 5.8 Trial2.square() print("圆的面积为: \(Trial2)") var Trial3 = 120.3 Trial3.square() print("圆的面积为: \(Trial3)")