设半径为R,此弦所对的圆心角等于A.
则此弦长L=（1/2）*R=R/2
A=2*ARC SIN(L/(2*R))
=2*ARC SIN((R/2)/(2*R))
=2*ARC SIN(1/4)
=28.955度
=28.955*PI/180
=0.50536弧度