(x-2t)^2+y^2=(t-1)^2의 포락선을 구하는데, 답을 정확히 하고 싶어서 울프럼 알파를 쓰려는데 뭐라고 넣어야 할지 모르겠어요... 그냥 envelope라 하면 안되네요...