设直角边a=b=1,斜边c=√2,
设内切圆半径为r,外接圆半径为R,
则r=(a+b-c)/2=(1+1-√2)/2=(2-√2)/2,
R=c/2=√2/2,
d/D=r/R=[(2-√2)/2]/(√2/2)
=√2-1.