Rất chi tiết nha cậu. 

#include <iostream>
#include <math.h>
using namespace std;
#define pi M_PI
int main()
{
    cout<<"Tinh dien tich hinh:\n1: Thang\n2: Tron\n3: Vuong\n4: Tam giac\n5: Chu nhat\nNhap so:";
    int n;
    cin>>n;
    if(n==1){
        int a,b,h;
        cout<<"Nhap chieu dai hai canh day, chieu cao hinh thang (a,b,h): ";
        cin>>a>>b>>h;
        cout<<"Dien tich hinh thang: "<<(float)((a+b)/2)*h;
    }
    else if(n==2){
        int r;
        cout<<"Nhap ban kinh hinh tron: ";
        cin>>r;
        cout<<"Dien tich hinh tron: "<<(float)(r*r)*pi;
    }
    else if(n==3){
        int a;
        cout<<"Nhap chieu dai 1 canh hinh vuong: ";
        cin>>a;
        cout<<"Dien tich hinh vuong: "<<a*a;
    }
    else if(n==4){
        int s,h,d;
        cout<<"Nhap chieu dai canh day, chieu cao hinh tam giac: ";
        cin>>d>>h;
        cout<<"Dien tich hinh tam giac: "<<(float)d*h/2;
    }
    else if(n==5){
        int a,b;
        cout<<"Nhap chieu dai, chieu rong hinh chu nhat: ";
        cin>>a>>b;
        cout<<"Dien tich hinh chu nhat: "<<a*b;
    }
    //samon247
    return 0;
}