;; The first three lines of this file were inserted by DrScheme. They record metadata
;; about the language level of this file in a form that our tools can easily process.
#reader(lib "htdp-beginner-reader.ss" "lang")((modname area_of_ring) (read-case-sensitive #t) (teachpacks ((lib "testing.ss" "teachpack" "htdp"))) (htdp-settings #(#t constructor repeating-decimal #f #t none #f ((lib "testing.ss" "teachpack" "htdp")))))
;; Write a contract, purpose, example, and test for the 
;; function below

;; define constant PI
(define PI 3.14)

;; Contract: area-of-disk : number -> number

;; Purpose: to compute the area of a disk whose radius is r

(define (area-of-disk r) 
  (* PI (* r r)))

;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;; Contract: area-of-ring : number number  ->  number

;; Purpose: to compute the area of a ring whose radius is
;; outer and whose hole has a radius of inner

;; Example: (area-of-ring 5 3) should produce 50.24

;; Definition: [refines the header]
(define (area-of-ring outer inner)
  (- (area-of-disk outer)
     (area-of-disk inner)))
  
;; Tests:
(area-of-ring 5 3) 
;; expected value
50.24