11.5.2

#lang racket

(define PI 3.14)

(define (add m n)
 (cond
  [(zero? m) n]
  [(zero? n) m]
  [else (add (add1 m) (sub1 n))]))

(define (multiply-by-pi n)
 (cond
  [(zero? n) 0]
  [(eq? n 1) PI]
  [else (+ PI (multiply-by-pi (sub1 n)))]))

(define (multiply m n)
 (cond
  [(or (zero? m) (zero? n)) 0]
  [(eq? m 1) n]
  [(eq? n 1) m]
  [else (add )]))

(multiply-by-pi 3)
(multiply-by-pi 4)
(multiply-by-pi 8)

(multiply 2 4)
(multiply 42 4)
(multiply 3 5)
(multiply 9 32)