设平均数为m
∵方差S^2=1/5[(X1-m)^2+(X2-m)^2+(X3-m)^2+(X4-m)^2+(X5-m)^2]
=1/5[(X1^2+X2^2+...+X5^2)-5m^2]
∴2=1/5[(X1²+X2²+X3²+X4²+X5²)-5*3^2]
故X1²+X2²+X3²+X4²+X5²=55