设弧半径为R,弧所对的圆心角为A,则有：
R^2=(R-H)^2+(L/2)^2
R^2=R^2-2*R*H+H^2+L^2/4
2*R*H=H^2+L^2/4
R=H/2+L^2/(8*H)
=1.3/2+4.5^2/(8*1.3)
=2.597米
A=2*ARC SIN((L/2)/R)
=2*ARC SIN((4.5/2)/2.597)
=120.07度
弧长C=PI*R*A/180=PI*2.597*120.07/180=5.443米