//ve khung chieu dai b - a. rong 3 hang. 
void KhungChuNhat(int a, int b)
 { 	
	int i; 	
	for (i = a; i <= b; i++) 	
	{ 		
		gotoxy(i, 17); 		printf("\xcd"); 		
		gotoxy(i, 19); 		printf("\xcd"); 	
	}  	
	gotoxy(a, 17); 	
	printf("\xc9"); 	
	gotoxy(a, 18); 		printf("\xba"); 	
	gotoxy(a, 19); 		printf("\xc8"); 	
	gotoxy(b, 17); 		printf("\xbb"); 	
	gotoxy(b, 18); 		printf("\xba"); 	
	gotoxy(b, 19); 		printf("\xbc"); 
}