(1-1/2的平方)(1-1/3的平方)(1-1/4的平方)······(1-1/N的平方)
=(1+1/2)(1-1/2)(1+1/3)(1-1/3)(1+1/4)(1-1/4)······(1+1/N)(1-1/N)
=3/2*1/2*4/3*2/3*5/4*3/4*······(1+N)/N*(N-1)/N
=(N+1)/(2N)
(1-1/2的平方)(1-1/3的平方)(1-1/4的平方)······(1-1/N的平方)
=(1+1/2)(1-1/2)(1+1/3)(1-1/3)(1+1/4)(1-1/4)······(1+1/N)(1-1/N)
=3/2*1/2*4/3*2/3*5/4*3/4*······(1+N)/N*(N-1)/N
=(N+1)/(2N)