SICP/1/1/7.scm

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

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

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

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

(define (good-enough? guess x)
  (< (abs (- (square guess)
             x))
     0.001))

(define (my-good-enough? guess prev-guess)
  (< (abs (- guess prev-guess))
     (/ guess 100000)))

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

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

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