diff --git a/ex-3_50-62.scm b/ex-3_50-62.scm
index 0bbf4ae..3bee6df 100644
--- a/ex-3_50-62.scm
+++ b/ex-3_50-62.scm
@@ -258,8 +258,25 @@
 
 (display "\nex-3.61 - invert-unit-series\n")
 
-(define (invert-unit-series s) 0)
+(define (invert-unit-series s)
+    (cons-stream 1 (mul-series (stream-map - (stream-cdr s))
+                               (invert-unit-series s))))
 
+(define X (invert-unit-series cosine-series))
 
-(display "\nex-3.62\n")
+(assert (sum (take 10 (mul-series cosine-series X)))
+        1)
+
+(display "\nex-3.62 - tan\n")
+
+(define (div-series ns ds)
+  (if (= (stream-car ds) 0)
+    (error "denominator is zero -- DIV-SERIES" ds)
+    (mul-series ns (invert-unit-series ds))))
+
+(define tan-series (div-series sine-series cosine-series))
+
+(assert (< (abs (- (exact->inexact (sum (take 20
+                                       (div-series sine-series cosine-series))))
+           1.5574)) 0.001) #t)