Added my solutions so far

ex-1-17.rkt

@@ -0,0 +1,17 @@
+#lang racket
+
+;; we are told to assume these are already defined.
+(define (double x)
+  (+ x x))
+(define (halve x)
+  (/ x 2))
+
+;; multiplication. defined in terms of addition, double and halve.
+;; logarithmic time, constant space.
+(define (mult x y)
+  (define (recc x y a)
+    (cond
+      ((= y 0) a)
+      ((even? y) (recc (double x) (halve y) a))
+      (else (recc x (- y 1) (+ a x)))))
+  (recc x y 0))