乘积 (1-2的平方分之一)(1-3的平方分之一)...(1-2000的平方分之一)
=(1+1/2)(1-1/2)(1+1/3)(1-1/3)...(1+1/2000)(1-1/2000)
=(1+1/2)(1+1/3)...(1+1/2000)(1-1/2)(1-1/3)...(1-1/2000)
=(3/2)(4/3)...(2001/2000)(1/2)(2/3)...(1999/2000)
=(2001/2)1/2000)=2001/4000.