已知弦长L=3.9米，拱高H=0.75米，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) =0.75/2+3.9^2/(8*0.75) =2.91米A=2*ARC COS((R-H)/R) =2*ARC COS((2.91-0.75)/2.91) =84.15度C=π*R*A/180=π*2.91*84.15/180=4.2739米
