设两条对角线长为X1、X2，则X1+X2=L,X1X2/2=S边长^2=(X1/2)^2+(X2/2)^2=1/4(X1^2+X2^2)=1/4[(X1+X2)^2-2X1X2] =1/4(L²-4S)∴边长=1/2根号L²-4S选 C
