弦长：L=9.5m.弦高：H=1.73m（从弦到弧的距离）.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)
=1.73/2+9.5^2/(8*1.73)
=7.386m
A=2*ARC SIN((L/2)/R)
=2*ARC SIN((9.5/2)/7.386)
=80.05度
=80.05*PI/180
=1.397112弧度
C=R*A=7.386*1.397112=10.319m