已知弦长L=100m拱高H=20m 半径R不知求弧长C?弧所对的圆心角为A.R^2=(R-H)^2+(L/2)^2R^2=R^2-2*R*H+H^2+L^2/42*R*H=H^2+L^2/4R=H/2+L^2/(8*H)=20/2+100^2/(8*20)=72.5mA=2*ARC SIN((L/2)/R)=2*ARC SIN((100/2)/72.5)=...