#| BEGIN (Write your solution here) |#
(define (minus-streams s1 s2)
  (stream-map - s1 s2))
(define (mul-series s1 s2)
  (cons-stream (* (stream-car s1)
                  (stream-car s2))
               (add-streams  (scale-stream (stream-cdr s1)
                                           (stream-car s2))
                             (mul-series s1
                                         (stream-cdr s2)))))



(define (invert-unit-series S)
  (cons-stream 1
               (scale-stream (mul-series (stream-cdr S)
                                          (invert-unit-series S))
                              -1)))
#| END |#