(define (sqrt-iter guess prev x)
  (if (good-enough? 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 prev)
  (< (abs (/ (- guess prev) prev)) 0.001))

(define (square-root x)
  (sqrt-iter 1.0 0.5 x))