(define (integrate-series s)
  (define (coefficient stream n)
    (cons-stream (/ (stream-car stream) n)
                 (coefficient (stream-cdr stream) (+ n 1))))
  (coefficient s 1))

(define cosine-series
  (cons-stream 1 (scale-stream (integrate-series sine-series) -1)))

(define sine-series
  (cons-stream 0 (integrate-series cosine-series)))

(define (mul-series a b)
  (cons-stream (* (stream-car a) (stream-car b))
               (add-streams (scale-stream (stream-cdr a)
                                          (stream-car b))
                            (mul-series a (stream-cdr b)))))