原式 = (1-1/2)(1+1/2)(1-1/3)(1+1/3)……(1-1/n)(1+1/n)
= 1/2 x 3/2 x 2/3 x 4/3 x 3/4 x 5/4 …… (n-1)/n x (n+1)/n
= 1/2 x (n+1)/n
= (n+1)/2n
因此极限为1/2