已知地面测量距离为L=66米（弦长）,地面到管道最深的距离是H=4米（弧弦之间的距离）.求管道的长度C（弧长）?
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)
=4/2+66^2/(8*4)
=138.125米
A=2*ARC SIN((L/2)/R)
=2*ARC SIN((66/2)/138.125)
=27.645度
=27.645*PI/180
=0.482495弧度
C=R*A=138.125*0.482495=66.645米