;; The first three lines of this file were inserted by DrRacket. 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 functions) (read-case-sensitive #t) (teachpacks ((lib "universe.ss" "teachpack" "2htdp"))) (htdp-settings #(#t constructor repeating-decimal #f #t none #f ((lib "universe.ss" "teachpack" "2htdp")))))
(define PI 3.14)

;; square number -> number
;; square a number x
(define (square x) 
  (* x x))

(check-expect (square 2) 4)
(check-expect (square 1) 1)

;; area-of-disk number -> number
;; calculate area of disk with radius r
(define (area-of-disk r) 
  (* PI (square r)))

(check-expect (area-of-disk 10) 314)

;; area-of-ring : number number -> number
;; compute the area of a ring with radius 
;; outer and hole radius of inner
(define (area-of-ring outer inner)
  (- (area-of-disk outer)
     (area-of-disk inner)))

(check-expect (area-of-ring 5 3) 50.24)