1-1/n^2=(n-1)/n*(n+1)/n
(1-1/4)(1-1/9).(1-1/n^2)
=1/2*3/2*2/3*4/3*3/4*5/4*……*(n-1)/n*(n+1)/n 中间项约去
＝(n+1)/2n
lim(1-1/4)(1-1/9).(1-1/n^2) n趋于无穷
=1/2