已知弦长L=20.5和拱高H=1.8求弧长C?弧半径为R,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)=1.8/2+20.5^2/(8*1.8)=30.084A=2*ARC SIN((L/2)/R)=2*ARC SIN((2...