(define (sqrt-iter guess guess-prev x) 
  (if (good-enough? guess guess-prev) 
      guess 
      (sqrt-iter (improve guess x) guess 
                 x))) 

(define (improve guess x)
  (average guess (/ x guess)))

(define (average x y)
  (/ (+ x y) 2))

(define (good-enough? guess guess-prev) 
  (< (abs (- guess guess-prev)) 0.001)) 

(define (sqrt-new x)
  (if (= x 0) x (sqrt-iter 1.0 999 x)))

(sqrt-new 16)
(sqrt-new 1.0004)
(sqrt-new 1)
(sqrt-new 134895724398)
(sqrt-new 0)