数据x1，x2满足|x1-x2|=a，则x1，x2的方差为
X1X2的方差
= {[X1-(X1+X2)/2]^2 + [X2-(X1+X2)/2]^2}/2
={[(X1-X2)/2]^2 + [(X2-X1)/2]^2}/2={(X1-X2)^2/2}/2
由于|x1-x2|=a
所以(X1-X2)^2 = a^2
X1、X2的方差=a^2/4
