問題1.8

(defun good-enough? (guess x)
  (< (abs (- (expt guess 3) x)) 0.001))

(defun improve (guess x)
  (/ (+ (/ x (expt guess 2)) (* guess 2)) 3))

(defun cbrt-iter (guess x)
  (if (good-enough? guess x)
      guess
      (cbrt-iter (improve guess x) x)))

(defun cbrt (x)
  (cbrt-iter 1.0 x))

二乗や三乗を求める関数の定義は自明なので、exptで。