Exercise 1.8 of SICP

(define (square x) (* x x))
    (define (great-enough? guess newguess)
      (< (abs (- 1 (/ guess newguess))) 0.0001))
    (define (improve guess x)
      (/ (+ (/ x (square guess)) (* 2 guess))
         3))
    (define (cube-root guess x)
      (if (great-enough? guess (improve guess x))
        guess
        (cube-root (improve guess x) x)))

> (cube-root 1.0 8)
2.000004911675504

> (cube-root 1.0 1e9)
1000.0162369748963