3) 

#include <iostream>
#include <cmath> 
using namespace std;
int main() {
    float a, b, c;
    cout << "Nhap ban kinh: ";
    cin >> a;
    b = 2 * M_PI * a;
    c = M_PI * a * a;
    cout << "Chu vi: " << b << endl;
    cout << "Dien tich: " << c << endl;

    return 0;
}

4)

#include <iostream>
using namespace std;
int main() {
    float toan, ly, hoa, a;
       cout << "Nhap diem mon Toan: ";
    cin >> toan;
    cout << "Nhap diem mon Ly: ";
    cin >> ly;
    cout << "Nhap diem mon Hoa: ";
    cin >> hoa;
    a = (toan + ly + hoa) / 3;
    cout << "Diem trung binh cua 3 mon la: " << a << endl;

    return 0;
}