Add 1.8

sicp-ex-1.8.rkt

@@ -0,0 +1,23 @@
+(define (square x) (* x x))
+
+(define (cubic-iter guess guess-prev x)
+  (if (good-enough? guess guess-prev)
+      guess
+      (cubic-iter (improve guess x) guess
+                  x))) 
+
+(define (improve guess x)
+  (average (* 2 guess) (/ x (square guess))))
+
+(define (average x y)
+  (/ (+ x y) 3))
+
+(define (good-enough? guess guess-prev) 
+  (< (abs (- guess guess-prev)) 0.001)) 
+
+(define (cubic-root x)
+  (if (= x 0) x (cubic-iter 1.0 999 x)))
+
+(cubic-root 1892379832.0)
+(cubic-root 8.0)
+(cubic-root 27.0)