SICP Exercise 1.9

(define (+ a b)
  (if (= a 0)
  b
  (inc (+ (dec a) b))))

(+ 4 5)
(inc (+ (dec 4) 5))
(inc (inc (+ (dec 3) 5)))
(inc (inc (inc (+ (dec 2) 5))))
(inc (inc (inc (inc (+ (dec 1) 5)))))
(inc (inc (inc (inc (inc (+ 0 5))))))

This process is recursive.

(define (+ a b)
  (if (= a 0)
  b
  (+ (dec a) (inc b))))

(+ 4 5)
(+ (dec 4) (inc 5))
(+ 3 6)
(+ (dec 3) (inc 6))
(+ 2 7)
(+ (dec 2) (inc 7))
(+ 1 8)
(+ (dec 1) (inc 8))
(+ 0 9)
9

This process is iterative.