Add Exercise 2.7

chapter2.lisp

@@ -56,3 +56,32 @@
 
 (defun print-point (p)
   (format t "(~a,~a)~%" (x-point p) (y-point p)))
+
+;; Exercise 2.7
+(defun make-interval (a b)
+  (cons a b))
+
+(defun lower-bound (x)
+  (car x))
+
+(defun upper-bound (x)
+  (cdr x))
+
+(defun add-interval (a b)
+  (make-interval (+ (lower-bound a) (lower-bound b))
+		 (+ (upper-bound a) (upper-bound b))))
+
+(defun mul-interval (a b)
+  (let ((p1 (* (lower-bound a) (lower-bound b)))
+	(p2 (* (lower-bound a) (upper-bound b)))
+	(p3 (* (upper-bound a) (lower-bound b)))
+	(p4 (* (upper-bound a) (upper-bound b))))
+    (make-interval (min p1 p2 p3 p4)
+		   (max p1 p2 p3 p4))))
+
+(defun div-interval (a b)
+  (mul-interval
+   a
+   (make-interval (/ 1.0 (upper-bound b))
+		  (/ 1.0 (lower-bound b)))))
+