sicp-ex-3.60

  
  
 (define (mul-series s1 s2) 
   (cons-stream (* (stream-car s1) (stream-car s2)) 
                (add-streams 
                 (scale-stream (stream-cdr s2) (stream-car s1)) 
                 (mul-series (stream-cdr s1) s2)))) 

  
  
 (define (mul-series s1 s2) 
   (cons-stream (* (stream-car s1) 
                   (stream-car s2)) 
                (add-streams (mul-streams (stream-cdr s1) 
                                          (stream-cdr s2)) 
                             (mul-series s1 s2)))) 
  
 (define (mul-series2 s1 s2) 
   (partial-sums (mul-streams s1 s2))) 
  

  
  
 (define (mul-series s1 s2) 
   (cons-stream 
    (* (stream-car s1) (stream-car s2)) 
    (add-streams (add-streams 
                  (scale-stream (stream-cdr s1) (stream-car s2)) 
                  (scale-stream (stream-cdr s2) (stream-car s1))) 
                 (cons-stream 0 
                              (mul-series (stream-cdr s1) 
                                          (stream-cdr s2)))))) 
  

atupal is correct

meteorgan and atupal are correct and zzd3zzd is wrong according to my checker.

  
 (define (display-stream-until n s)     ; n-th value included 
   (if (< n 0) 
       the-empty-stream 
       (begin (newline) (display (stream-car s)) 
              (display-stream-until (- n 1) (stream-cdr s))))) 
  
 ; It can be easily seen that (mul-series (stream-cdr s1) s2) equals 
 ; (add-streams (scale-stream (stream-cdr s2) (stream-car s1)) 
 ;              (cons-stream 0 (mul-series (stream-cdr s1) (stream-cdr s2)))) 
 ; , which means that meteorgan's solution and atupal's solution are equivalent. 
  
 (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 (stream-cdr s2) s1)))) 
  
 (define circle-series 
   (add-streams (mul-series cosine-series cosine-series) 
                (mul-series sine-series sine-series))) 
  
 ; if you see one 1 followed by 0's, your mul-series is correct. 
 (display-stream-until 30 circle-series) 
  
  

I agree with seok. However, in testing with non-infinite series I found that the book's version of stream-map does not cope with series of different lengths, and so gives an incorrect result for mul-series applied to non-infinite series. See http://community.schemewiki.org/?sicp-ex-3.50 for my fix for this.

  
  
 (define (mul-series s1 s2) 
   (cons-stream (* (stream-car s1) (stream-car s2)) 
                (add-streams 
                 (scale-stream (stream-cdr s2) (stream-car s1)) 
                 (mul-series (stream-cdr s1) s2))))