已知弦长L，弦高H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*H*R+H^2+L^2/42*H*R=H^2+L^2/4R=H/2+L^2/(8*H)A=2*ARC SIN((L/2)/R)C=PI*R*A/180R=H/2+L^2/(8*H)A=2*ARC SIN((L/2)/R)C=PI*R*A/180
