(defn width [i]
(/ (- (upper-bound i) (lower-bound i)) 2))
+
+;; parallel-resistor formula
+(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)))))