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FOCI.LSP -- Locate and mark the foci of AutoCAD ELLIPSE objects.

  (c) 1998 Tee Square Graphics.

  Creates a POINT object at each focus of each selected ELLIPSE;
  user may set point display mode (PDMODE) and size (PDSIZE) in
  accordance with personal preferences. Use the NODE Object Snap
  to select foci for purposes of dimensioning and other operations.

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(defun c:foci (/ ss i en c ep m a n h)
  (setq ss (ssget '((0 . "ELLIPSE")))
        i 0)
  (while (and ss (< i (sslength ss)))
    (setq en (entget (ssname ss i))
          c (cdr (assoc 10 en))
          ep (cdr (assoc 11 en))
          m (distance '(0 0 0) ep)
          a (angle '(0 0 0) ep)
          n (* m (cdr (assoc 40 en)))
          h (sqrt (- (* m m)(* n n)))
          i (1+ i))
    (entmake (list '(0 . "POINT")(cons 10 (polar c a h))))
    (entmake (list '(0 . "POINT")(cons 10 (polar c a (- h))))))
  (princ)
)