用平方差
=(1-1/2)(1+1/2)(1-1/3)(1+1/3)……(1-1/2013)(1+1/2013)
=(1/2)(3/2)(2/3)(4/3)……(2012/2013)(2014/2013)
中间约分
=(1/2)(2014/2013)
=1007/2013