#include <iostream>
using namespace std;

#define pi 3.14
#include <cmath>
#include <conio.h>

float STron(float bankinh)
{
	return pi * bankinh * bankinh;
}

float SHCN(float dai, float rong)
{
	return dai * rong;
}

float SHV(float canh)
{
	return canh * canh;
}

float NuaChuViTamGiac(float a, float b, float c)
{
	return (a + b + c) / 2;
}

float STamGiac(float a, float b, float c)
{
	float P = NuaChuViTamGiac(a, b, c);
	return sqrt( P * (P - a) * (P - b) * (P - c) );
}

float SBinhHanh(float a, float h)
{
	return a * h;
}

int main()
{
	BatDau:

	cout << "\n1. Tinh dien tich hinh tron.";
	cout << "\n2. Tinh dien tich hinh chu nhat.";
	cout << "\n3. Tinh dien tich hinh vuong.";
	cout << "\n4. Tinh dien tich hinh tam giac.";
	cout << "\n5. Tinh dien tich hinh binh hanh.";

	int luachon;
	cout << "\n\nNhap lua chon cua ban: ";			cin >> luachon;

	float KQ;
	switch (luachon)
	{
		case 1:
		{
			float bankinh;
			cout << "\nNhap ban kinh hinh tron: ";		cin >> bankinh;
			KQ = STron(bankinh);
			cout << "\nKQ = " << KQ;
			break;
		}

		case 2:
		{
			float dai, rong;
			cout << "\nNhap chieu dai & chieu rong: ";		cin >> dai >> rong;
			KQ = SHCN(dai, rong);
			cout << "\nKQ = " << KQ;
			break;
		}

		case 3:
		{
			float canh;
			cout << "\nNhap chieu dai canh hinh vuong: ";		cin >> canh;
			KQ = SHV(canh);
			cout << "\nKQ = " << KQ;
			break;
		}

		case 4:
		{
			float a, b, c;
			cout << "\nNhap chieu dai 3 canh a, b & c: ";		cin >> a >> b >> c;
			KQ = STamGiac(a, b, c);
			cout << "\nKQ = " << KQ;
			break;
		}

		case 5:
		{
			float a, h;
			cout << "\nNhap canh day & chieu cao: ";		cin >> a >> h;
			KQ = SBinhHanh(a, h);
			cout << "\nKQ = " << KQ;
			break;
		}
		default: cout << "Lua chon chua chinh xac";
	}

	cout << "\nNhap y de kiem tra lai:";
	char y = getch();
	if(y == 'Y' || y == 'y')
	{
		system("cls");
		goto BatDau;
	}

	system("pause");
	return 0;
}