题目：弦长L=12m,弦中心到弧高H=0.9m,计算弧长C多少?
设 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+12^2/(8*0.9)
=20.45
A=2*ARC SIN((L/2)/R)
=2*ARC SIN((12/2)/20.45)
=34.123度
=34.12*3.14/180
=0.5956弧度
C=A*R
=0.5956*20.45 =12.18m