double tinhNGThua(int n);
case 7:
{
	int n;
	printf("nhap so nguyen n: ");
	scanf_s("%d", &n);

	if (n < 0) 
		printf("Khong co giai thua cho so am!\n");
	else 
		printf("%d! = %.2lf\n", n, tinhNGThua(n));
	break;
}
double tinhNGThua(int n)
{
	if (n == 0 || n == 1)
		return 1;
	else
		return n * tinhNGThua(n - 1);
}