记B1={第次结成绳圈},则 A B i=1 P(B1)=2n(2 (2n)! P(Bk+1B1B)=2(n-k)-1 由求法公式 P(A)= 12(n-)-1=2n=1m❫❻❱➬❺Ú❖Õá ❫❻❱➬ ➥❻Õá ➪✟✛Õá✺ PBi = {✶i❣✭↕✲✗} ➜❑ A = \n i=1 Bi P(B1) = 2n(2n − 2)! (2n)! = 1 2n − 1 P(Bk+1|B1 · · · Bk) = 1 2(n − k) − 1 ❞➛④ú➟ P(A) = nY−1 k=0 1 2(n − k) − 1 = 1 (2n − 1)!!