4+5+6+X+Y=5*5
X+Y=10
[(5-4)^2+(5-6)^2+(X-5)^2+(Y-5)^2]/5=4/5
(X-5)^2+(Y-5)^2=2
X^2-10X+25+Y^2-10Y+25=2
X^2+Y^2-10(X+Y)=-48
X^2+Y^2=10*10-48=52
(X+Y)^2=X^2+Y^2+2XY=100
2XY=100-52=48
(X-Y)^2=X^2+Y^2-2XY=52-48=4
故有|X-Y|=2