(/ 1.0 (lower-bound y)))))
+;; provide new constructors
+(defn make-center-width [c w]
+ (make-interval (- c w) (+ c w)))
+
+(defn center [i]
+ (/ (+ (lower-bound i) (upper-bound i)) 2))
+
+(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)))))