SICP 1.7

(define (square x) (* x x))

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

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

;;; Newly implemented guess
;;; Good enough when |guess - improved-guess|/guess < 0.001 (0.1%)
(define (good-enough? guess improved-guess x)
  (< (/ (abs (- guess improved-guess)) guess) 0.001))

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

(define (sqrt x)
  (sqrt-iter 1.0 x))