SICP1.7

(define (sqrt-iter guess x) 
    (if (good-enough? guess (improve 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 (- 1 (/ guess x))) 0.0001))

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

(print (sqrt 9))
(print (sqrt 0.000001))
(print (sqrt 9000000000000))