用平方差
=(1-1/2)(1+1/2)(1-1/3)(1+1/3)……(1-1/2014)(1+1/2014)
=(1/2)(3/2)(2/3)(4/3)……(2013/2014)(2015/2014)
中间约分
=(1/2)(2015/2014)
=2015/4028