보통은, 진동할 수도 있고, 1+2- 2+3- 3과 같은 것들이 있을 수도 있습니다 이렇게 해서 중간항들은 모두 사라지고
Normally, you would have things like, maybe they oscillate, maybe you have stuff like, you know, 1 plus 2, minus 2, plus 3, minus 3, and so all the middle terms cancel out, and you just have the first term and the last term left.QED QED