First Racket program to calculate the content size of a ring

area-of-ring.rkt

;; area-of-ring: number number -> number 
;; Determines the area of a ring
;; with outer radius outer and an inner radius inner
;; Example: (area-of-ring 5 3) is
;; roughly 50.26544
(define (area-of-ring outer inner)
  (- (area-of-circle outer)
     (area-of-circle inner)))

;;Tests
(check-expect (area-of-ring 0 0) 0)
(check-within (area-of-ring 5 5) 0 0.0001)
(check-within (area-of-ring 5 3) 50.26544 0.001)

;; area-of-circle: number -> number
;; Determines the area of a circle with a radius radius
;; Example: (area-of-circle 1) is pi (3.14159)
(define (area-of-circle radius)
  (* radius radius pi))

;; Tests
(check-expect (area-of-circle 0) 0)
(check-within (area-of-circle 4) (* 16 pi) 0.0001)