原式=(1-1/2)(1+1/2)(1-1/3)(1+1/3)……(1-1/n)(1+1/n)
=(1/2)(3/2)(2/3)(4/3)……[(n-1)/n][(n+1)/n]
=(1/2)[(n+1)/n]
=(n+1)/2n
=n/2n+1/2n
=1/2+1/2n>1/2