; (= (* 2 c) (/ (- (* 3 (* a b)) (* 5 (* (^ c 2) d)))  (- (* 2 (* a c)) (* 6 (* (^ x 2) y)))))

(defun ocurre (X Expr)
  (if (eq X Expr)
      T
      (if (listp Expr)
          (or (ocurre X (second Expr))
              (ocurre X (third Expr))
          )
          NIL
      )
  )
)

(defun resuelve (X Ecuacion)
  (if (eq X (second Ecuacion))
      Ecuacion
      (let* ( (L-Izq (second Ecuacion))
              (L-Der (third Ecuacion))
              (Op (first L-Izq))
              (A  (second L-Izq))
              (B  (third L-Izq))
            )
         (cond ( (eq Op '+)  ; A + B = L-Der ; (= (+ A B) L-Der)
                 (if (ocurre X A) ; A = L-Der - B ; (= A (- L-Der B))
                     (resuelve X (list '= A (list '- L-Der B)))
                     (resuelve X (list '= B (list '- L-Der A)))
                 )
               )
               
               ( (eq Op '*)  
                 (if (ocurre X A) 
                     (resuelve X (list '= A (list '/ L-Der B)))
                     (resuelve X (list '= B (list '/ L-Der A)))
                 )
               )
               ; ... resta, multiplicacion, etc.
         )
      )              
  )
)

    Source: geocities.com/mx/mticucea

               ( geocities.com/mx)