Add 1.21

sicp-ex-1.21.rkt

@@ -0,0 +1,17 @@
+(define (square n)
+  (* n n))
+
+(define (smallest-divisor n)
+  (find-divisor n 2))
+
+(define (find-divisor n test-divisor)
+  (cond ((> (square test-divisor) n) n)
+        ((divides? test-divisor n) test-divisor)
+        (else (find-divisor n (+ test-divisor 1)))))
+
+(define (divides? a b)
+  (= (remainder b a) 0))
+
+(smallest-divisor 199)
+(smallest-divisor 1999)
+(smallest-divisor 19999)