5.9打卡

#include <bits/stdc++.h>
using namespace std;
const double PI=3.14159265358979;
inline double calArea(double radius)
{
    return PI*radius*radius; 
}
int main()
{
    double r=3.0;
    double area=calArea(r);
    cout<<area<<endl;
    return 0;
}