C++题目，跪求高手解答，谢谢

Question

编程定义一个Circle类，含有私有变量半径r，构造函数能够初始化r,成员函数能计算圆面积，然后使用该类来计算并显示一个圆环的面积，其中小圆半径为4.2，大圆半径为8.6。

Answer 1

#define Pi 3.1415926
#include<iosteram.h>
class Circle
{
public:
Circle(float r)
{this->r=r;}
float csquare();
private:
float r;
};
float Circle::csquare()
{float S;
S=Pi*r*r;
return S;}
void main()
{
float s1;
Circle C1(4.2),C2(8.6);
s1=C2.csquare-C1.csquare;
cout<<"圆环的面积是："<<s1<<endl;
}

Answer 2

不懂

Answer 3

去买一本 谭浩强 的c语言 第三版

Answer 4

Circle big(8.6),small(4.2);
big.area()-small.area();

Answer 5

按照lz要求的代码，已在VC6编译通过：enjoy!

#include <iostream>
using namespace std;

#define PI 3.1415926

class Circle
{
public:
Circle(double a)
{
r = a;
};
private:
double r;

public:
double Area()
{
return PI * r * r;
}
};

void main()
{
Circle c1(4.2);
Circle c2(8.6);

cout << "small circle area: " << c1.Area() << endl;
cout << "big circle area: " << c2.Area() << endl;

}

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