已知弦长L=400cm,弦高H=77cm（弦的中点到弧的距离）,求弧长C?
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*H*R+H^2+L^2/4
2*H*R=H^2+L^2/4
R=H/2+L^2/(8*H)
=77/2+400^2/(8*77)
=298.24cm
A=2*ARC SIN((L/2)/R)
=2*ARC SIN((400/2)/298.24)
=84.23度
=84.23*PI/180
=1.470035弧度
C=R*A=298.24*1.470035=438.42cm