=(1-1/2)(1+1/2)(1-1/3)(1+1/3)(1-1/4).(1+1/9)(1-1/10)(1+1/10) =1/2*3/2*2/3*4/3*3/4*.*10/9*9/10*11/10 除首尾2项,其余均约去 则=1/2*11/10 =11/20