吧友的贴，有计算弧长的LISP代码。
https://tieba.baidu.com/p/6939455524?share=9105&fr=sharewise&see_lz=0&share_from=post&sfc=copy&client_type=2&client_version=12.57.5.0&st=1713017561&is_video=false&unique=DE8E8B586409C0CC9A80E5F99F06135D

吧友的代码，用弧长画弧。
(defun c:AAA (/ p1 p2 s a l x xx fx flx r c c1 c2 mspace myobj)
(defun N ()
(setq c (- (+ a (* 0.5 pi)) (* x 0.5)))
(setq cen (polar p1 c r))
(setq c1 (+ c pi))
(setq c2 (+ c1 x))
(setq myobj (vla-addarc mspace (vlax-3d-point cen) r c1 c2))
(princ)
)
(setq p1 (getpoint "\n请输入圆弧第一点:"))
(setq p2 (getpoint p1 "\n请输入圆弧第二点:"))
(setq s (getdist p1 "\n请输入弧长："))
(setq a (angle p1 p2))
(setq l (distance p1 p2))
(vl-load-com)
(setq mspace (vla-get-modelspace
(vla-get-activedocument (vlax-get-acad-object))
)
)
(if (<= s l)
(progn
(prompt "您所要画的圆弧并不存在！")
(princ)
)
(progn
(setq x 2)
(setq fx (- (/ (sin (/ x 2)) x) (/ (* 0.5 l) s)))
(setq
flx (/ (- (* 0.5 x (cos (* 0.5 x))) (sin (* 0.5 x))) (* x x))
)
(setq xx (- x (/ fx flx)))
(while (> (abs (- x xx)) 0.0000000001)
(setq x xx)
(setq fx (- (/ (sin (/ x 2)) x) (/ (* 0.5 l) s)))
(setq flx
(/ (- (* 0.5 x (cos (* 0.5 x))) (sin (* 0.5 x))) (* x x))
)
(setq xx (- x (/ fx flx)))
)
(setq r (/ s xx))
(initget "N S")
(setq aa (getkword "\n 请输入圆弧方向[逆时针(N)/顺时针(S)]:"))
(if (= aa nil)
(setq aa "N")
)
(if (= aa "N")
(N)
(if (= aa "S")
(progn
(setq c (- (+ a (/ x 2)) (* 0.5 pi)))
(setq cen (polar p1 c r))
(setq c1 (- (+ c pi) x))
(setq c2 (+ c pi))
(setq myobj (vla-addarc mspace (vlax-3d-point cen) r c1 c2))
(princ)
)
)
)
)
)
)

cad新手入门练习https://tieba.baidu.com/p/763325850?pn=1