原式=(1-1/2)(1+1/2)(1-1/3)(1+1/3)(1-1/4)(1+1/4)...(1-1/2013)(1+1/2013)=(1/2)(3/2)(2/3)(4/3)(3/4)(5/4)...(2012/2013)(2014/2013)=(1/2)(2014/2013)=1007/2013