用平方差
=(1-1/5)(1+1/5)(1-1/6)(1+1/6)(1-17)(1+1/7)……(1-1/200)(1+1/200)
=(4/5)(6/5)(5/6)(7/6)(6/7)(8/7)……(199/200)(201/200)
中间约分
=(4/5)(201/200)
=201/250