Added my solutions so far

ex-2.rkt

@@ -0,0 +1,88 @@
+#lang racket
+
+(define (mul-interval x y)
+  (let ((p1 (* (lower-bound x) (lower-bound y)))
+        (p2 (* (lower-bound x) (upper-bound y)))
+        (p3 (* (upper-bound x) (lower-bound y)))
+        (p4 (* (upper-bound x) (upper-bound y))))
+    (make-interval (min p1 p2 p3 p4)
+                   (max p1 p2 p3 p4))))
+
+(define (make-interval a b) (cons a b))
+
+(define (add-interval a b)
+  (make-interval (+ (lower-bound a) (lower-bound b))
+                 (+ (upper-bound a) (upper-bound b))))
+
+;; 2.7
+;; simple definitions.
+(define upper-bound cdr)
+(define lower-bound car)
+
+
+;; 2.8
+(define (interval-diff f a b)
+  (make-interval (- (lower-bound a) (upper-bound b))
+                 (- (upper-bound a) (lower-bound b))))
+
+
+;; 2.9 this one's just about reasoning, no programming, and I feel too lazy to explain
+;; sorry.
+
+;; 2.10 : simple 'nuff.
+(define (div-interval x y)
+  (if (= (lower-bound y) (upper-bound y))
+      (error "WHY? WHY MUST YOU TORTURE ME SO?")
+      (mul-interval
+       x
+       (make-interval (/ 1.0 (upper-bound y))
+                      (/ 1.0 (lower-bound y))))))
+
+;; 2.11 : This one's also somewhat simple.
+;; we ONLY need more than two multiplications
+;; if BOTH intervals have different signs on their upper
+;; and lower bounds. otherwise, naive logic works fine.
+
+(define (mul-interval2 x y)
+  (define (different? n)
+    (and (< (lower-bound n) 0) (< (upper-bound n) 0)))
+  (if (and (different? x) (different? y))
+      (let ((p1 (* (lower-bound x) (lower-bound y)))
+            (p2 (* (lower-bound x) (upper-bound y)))
+            (p3 (* (upper-bound x) (lower-bound y)))
+            (p4 (* (upper-bound x) (upper-bound y))))
+        (make-interval (min p1 p2 p3 p4)
+                       (max p1 p2 p3 p4)))
+      (make-interval (* (lower-bound x) (lower-bound y))
+                     (* (upper-bound x) (upper-bound y)))))
+
+;; 2.12 : simple nuff.
+(define (make-center-width c w)
+  (make-interval (- c w) (+ c w)))
+(define (center i)
+  (/ (+ (lower-bound i) (upper-bound i)) 2))
+(define (width i)
+  (/ (- (upper-bound i) (lower-bound i)) 2))
+(define (make-center-percent c p)
+  (make-center-width c (* c (/ p 100))))
+(define (percent i)
+  (* (/ (width i) 2) 100))
+
+;; 2.13 & 2.14
+(define (par1 r1 r2)
+  (div-interval (mul-interval r1 r2)
+                (add-interval r1 r2)))
+(define (par2 r1 r2)
+  (let ((one (make-interval 1 1)))
+    (div-interval
+     one (add-interval (div-interval one r1)
+                       (div-interval one r2)))))
+
+(define A (make-interval 100 101))
+(define B (make-interval 200 201))
+
+(div-interval A A)
+(div-interval (make-interval 1 1) A)
+(div-interval B A)
+
+