what is wrong with this

I try to write a code to get the arc length between any two points on it.

But my answer is wrong . why

( defun arclength()
 (setq ss (car (entsel "\nSelect arc:")))
 (setq x_object (vlax-Ename->Vla-Object ss))
 (setq r (vla-get-radius ename))
 (setq dist(getdist"/ pick on arc"))
 (setq dist2 (/ dist 2))
 (setq radian (atan  dist2 r))
 (setq distarc (* 2 r radian ))
 )

Try this:

(defun arclength ( / en di rd )
   (if
       (and
           (setq en (car (entsel "\nSelect Arc: ")))
           (= "ARC" (cdr (assoc 0 (entget en))))
           (setq di (getdist "\nPick Chord: "))
           (setq rd (cdr (assoc 40 (entget en))))
       )
       (* 2.0 rd (asin (/ di 2.0 rd)))
   )
)

;; ArcSine  -  Lee Mac
;; Args: -1 <= x <= 1

(defun asin ( x )
   (if (<= -1.0 x 1.0)
       (atan x (sqrt (- 1.0 (* x x))))
   )
)

ArcSine from here.

O.K Thanks it is working nicelly

You're welcome