Subdividing a Polygon Parallel to a Line

(defun ang_between (p10 p11 p20 p21 / px p1 p2 l_pt l_d p ang)
(setq px (inters p10 p11 p20 p21 nil))
(cond
(px
(if (> (distance px p10) (distance px p11)) (setq p1 p10) (setq p1 p11))
(if (> (distance px p20) (distance px p21)) (setq p2 p20) (setq p2 p21))
(setq
l_pt (list px p1 p2)
l_d (mapcar 'distance l_pt (append (cdr l_pt) (list (car l_pt))))
p (/ (apply '+ l_d) 2.0)
ang (* (atan (sqrt (/ (* (- p (car l_d)) (- p (caddr l_d))) (* p (- p (cadr l_d)))))) 2.0)
)
)
(T
nil
)
)
)
(defun c:divpar ( / pt1 pt2 pt3 pt4 S1 ang1 ang2 x1 x2 ptx1 ptx2)
  (setq pt1 (getpoint "\nFirst point of baseline: "))
  (setq pt2 (getpoint pt1 "\nSecond point of baseline: "))
  (setq pt3 (getpoint pt1 "\nPoint of first adjacent side: "))
  (setq pt4 (getpoint pt2 "\nPoint of second adjacent side: "))
  (setq S1 (getreal "\nWanted area: "))
  (setq ang1 (ang_between pt1 pt2 pt1 pt3))
  (setq ang2 (ang_between pt2 pt1 pt2 pt4))
  (setq ang1 (- pi ang1) ang2 (- pi ang2))
  (setq x1
    (*
      (/
        (* (distance pt1 pt2) (sin ang1))
        (sin (+ ang1 ang2))
      )
      (1-
        (+ ;or can be "-"
          (sqrt
            (1+
              (/
                (* 2.0 S1 (sin (+ ang1 ang2)))
                (* (distance pt1 pt2) (distance pt1 pt2) (sin ang1) (sin ang2))
              )
            )
          )
        )
      )
    )
  )
  (setq x2 (/ (* x1 (sin ang2)) (sin ang1)))
  (setq ptx1 (polar pt1 (angle pt1 pt3) x2))
  (setq ptx2 (polar pt2 (angle pt2 pt4) x1))
  (command "_.line" "_none" ptx1 "_none" ptx2 "")
)