L=18米跨度，H=2米高拱形，求弧长C? 弧半径为R，弧所对的圆心角为A。R=H/2+L^2/(8*H) =2/2+18^2/(8*2) =21.25米A=2*ARC COS((R-H)/R) =2*ARC COS((21.25-2)/21.25) =50.115度C=π*R*A/180=π*21.25*50.115/180=18.5869米
