L=38.75,H=5.5
R=(L^2+4*H^2)/(8H)=(39.75^2+4*5.5^2)/(8*5.5)
=38.66
A=2*ARC SIN(L/(2*R))=2*ARC SIN(39.75/(2*38.66))=61.8743度=1.08弧度
弧长=R*A=41.749