1.8.scm

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

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

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

(define (average x y z) 
  (/ (+ x y z) 3))

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

(define (cube x) (* x x x))