#include <iostream>
#include <cmath>
using namespace std;
double area(double radius);
double volumn(double radius);
const double PI=3.14159;
int main()
{
    double radius, area, volumn;
    cout << "Please enter radius for both:" << radius << endl;
    cin >> radius;
    area = area(radius);
    volumn = volumn(radius);
    cout << "The area is " << area << endl
         << "The volumn is " << volumn << endl;
    return 0;
}
double area(double radius)
{
    return(PI * pow(radius,2));
}
double volumn(double radius)

{
    return((4.0/3.0) * PI * pow(radius,3));
}

#include <iostream>
#include <cmath>
using namespace std;
double area(double radius);
double volumn(double radius);
const double PI=3.14159;
int main()
{
    double radius, area, volumn;
    cout << "Please enter radius for both:" << radius << endl;//radius还没有初始化；
    cin >> radius;
    area = area(radius);//变量名和函数名重复了，把变量area改为area1;
    volumn = volumn(radius);//同上；
    cout << "The area is " << area << endl//别忘记改
         << "The volumn is " << volumn << endl;//同上
    return 0;
}
double area(double radius)
{
    return(PI * pow(radius,2));
}
double volumn(double radius)

{
    return((4.0/3.0) * PI * pow(radius,3));
}

哦，原来变量名和函数名不能重复啊！谢谢了啊～