Exercise 1.38

(define (approx-e k)
  (define (d i)
    (if (zero? (remainder (+ i 1) 3))
        (* 2/3 (+ i 1))
        1))
  (+ 2 (cont-frac always-one d k)))
(define (always-one i) 1.0)

(approx-e 1) ~> 3.0
(approx-e 2) ~> 2.6666666666666665
(approx-e 3) ~> 2.75
(approx-e 4) ~> 2.7142857142857144
(approx-e 5) ~> 2.71875
(approx-e 1000) ~> 2.7182818284590455

    (define (cont-frac n d k)
      (define (helper i)
        (if (= i k)
            0
            (/ (n i)
               (+ (d i) (helper (+ i 1))))))
      (helper 1))