Exercise 3.62.  Use the results of exercises 3.60 and 3.61  to define a 
procedure div-series that divides two power series. Div-series should 
work for any two series, provided that the denominator series begins 
with a nonzero constant term. (If the denominator has a zero constant 
term, then div-series should signal an error.) Show how to use 
div-series together with the result of exercise 3.59  to generate the 
power series for tangent.

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(define (div-series s1 s2)
  (if (eq? (stream-car s2) 0)
      (error "zero constant term in denominator")
      (let ((scale (/ 1 (stream-car s2))))
        (mul-series (scale-series s1 scale)
                    (invert-unit-series (scale-series s2 scale))))))

(define (tangent-series) (div-series cosine-series sine-series))