;; constant
(define PI 3.142)


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


;; area-circle : number -> number
(define (area-circle radius)
  (* PI (square radius)))


;; circumference-circle : number -> number
(define (circumference-circle radius)
  (* 2 PI radius))

   
;; area-cylinder-side : number number -> number
(define (area-cylinder-side radius length)
  (* (circumference-circle radius) length))

  
;; area-pipe : number number number -> number
;; A pipe has an inside surface and a outside surface
;; and a top and bottom surface.
(define (area-pipe radius length thickness)
  (+
   (+
    (area-cylinder-side radius length)
    (area-cylinder-side (+ radius thickness) length))
   (*
    2
    (-
     (area-circle (+ radius thickness))
     (area-circle radius)))))

;; area-pipe : number number number -> number
;; A pipe has an inside surface and a outside surface
;; and a top and bottom surface.
(define (area-pipe radius length thickness)
  (+
   (+
    (* (* 2 3.142 radius) length)
    (* (* 2 3.142 (+ radius thickness) length)))
   (*
    2
    (-
     (* 3.142 (* (+ radius thickness) (+ radius thickness)))
     (* 3.142 (* radius radius))))))