原式=(1-1/2)(1+1/2)(1-1/3)(1+1/3)(1-1/4)(1+1/4).(1-1/99)(1+1/99)(1-1/100)(1+1/100)
=(1/2)[(3/2)(2/3)(4/3)(3/4)(5/4).(98/99)(100/99)(99/100)](101/100)
=(1/2)*1*1*1.*1*(101/100)
=(1/2)(101/100)
=101/200为什么前面有个二分之一前面的1/2没有约掉，后面的101/100也没有约掉