solution for exercise 1.31

diff --git a/src/sicp/ex1_31.clj b/src/sicp/ex1_31.clj
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+(ns sicp.ex1_31
+  (:use [clojure.contrib test-is]
+       [sicp utils]))
+
+(defn product [a b]
+  (if (> a b)
+    1
+    (* a (product (+ a 1) b))))
+
+(defn prod [term a next b]
+  (if (> a b)
+    1
+    (* (term a)
+       (prod term (next a) next b))))
+
+;; factorial
+;; fact (n) = n * factorial (n - 1)
+
+(defn factorial [n]
+  (prod identity 1 inc n))
+
+(deftest test-factorial-of-6
+  (is (factorial 6) 720))
+
+(deftest test-factorial-of-10
+  (is (factorial 10) 3628800))
+
+(defn prod-pi [a b]
+  (prod #(/ (* % (+ % 2)) (square (+ % 1.0))) a #(+ % 2) b))
+
+(deftest test-pi-2-digits-accuracy
+  (is (> (Math/abs (- 3.14159 (* 4 (prod-pi 2 1000)))) 0.001) true))
+
+(defn iter-prod [term a next b result]
+  (if (> a b)
+    result
+    (iter-prod term (next a) next b (* a result))))
+
+(defn iprod [term a next b]
+  (iter-prod term a next b 1))
+
+(deftest test-iprod-for-prod-of-1-to-10
+  (is (iprod identity 1 inc 10) (prod identity 1 inc 10)))