2.1.4.6.clj

(defn par1 [r1 r2]  
  (div-interval (mul-interval r1 r2)  
                (add-interval r1 r2)))

(defn par2 [r1 r2]  
  (let [one (make-interval 1 1)]  
    (div-interval one  
                  (add-interval (div-interval one r1)  
                                (div-interval one r2)))))