正接 = tangent だから、sin/cos してやればいいだけですね。
(load "./chapter3.scm")
(load "./ex360.scm")
(load "./ex361.scm")
(define (div-series s1 s2)
(if (= 0 (stream-car s2))
(raise "constant term is 0")
(mul-series s1
(invert-unit-series s2))))
(define tangent-series
(div-series sine-series cosine-series))