知道底边L和高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*R*H+H^2+L^2/42*R*H=H^2+L^2/4R=H/2+L^2/(8*H)A=2*ARC SIN((L/2)/R)C=A*R*PI/180
