知道弦长L和高度H,求弧长C.
设弧半径是R,圆心角是A.
R=L^2/(8*H)+H/2
A=2*ARC SIN(L/(2*R))*PI/180
C=A*R
=(2*ARC SIN(L/(2*(L^2/(8*H)+H/2)))*PI/180)*(L^2/(8*H)+H/2)