弓高H=0.4米弦长L=4.2米求弧长C?
弧半径为R,弧所对的圆心角为A.
R^2=(R-H)^2+(L/2)^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)
=0.4/2+4.2^2/(8*0.4)
=5.7125米
A=2*ARC SIN((L/2)/R)
=2*ARC SIN((4.2/2)/5.7125)
=43.137度
C=π*R*A/180=π*5.7125*43.137/180=4.30009米