R. Akiva says: Whence is it derived that if beth-din saw him killing but did not recognize him, they would not break the neck? From (Ibid. 7) "And they shall answer and say … our eyes have not seen" (But in that instance their eyes did see.)
(Ibid. 2) "Then your elders shall go out": "your elders" — two; "and your judges" — two. And since a beth-din cannot be evenly balanced, an additional one is added, making five. These are the words of R. Yehudah. R. Shimon says: "Your elders and your judges" — two. And since a beth-din cannot be evenly balanced, an additional one is added, making three.