两点距离2.1米,中点垂直高0.9米,问过此3点的弧长怎么算?

两点距离L=2.1米,中点垂直高H=0.9米,问过此3点的弧长C怎么算?
弧半径为R,弧所对的圆心角为A.
R^2=(R-H)^2+(L/2)^2
R^2=R^2-2*R*H+H^2+L^2/4
2*R*H=H^2+L^2/4
R=H/2+L^2/(8*H)
=0.9/2+2.1^2/(8*0.9)
=1.0625米
A=2*ARC SIN((L/2)/R)
=2*ARC SIN((2.1/2)/1.0625)
=162.4度
C=PI*R*A/180=PI*1.0625*162.4/180=3.0117米