设A=1/2+1/4+1/6（1+1/2+1/4+1/6）×（1/2+1/4+1/6+1/8)-(1+1/2+1/4+1/6+1/8)(1/2+1/4+1/6)=(1+A)*(A+1/8)-(1-A-1/8)*A=A²+A+(1/8)A+1/8-A²-A-(1/8)A=1/8
