diff --git a/chapter2.lisp b/chapter2.lisp
index 1c8cce4..6a76738 100644
--- a/chapter2.lisp
+++ b/chapter2.lisp
@@ -85,3 +85,8 @@
    (make-interval (/ 1.0 (upper-bound b))
 		  (/ 1.0 (lower-bound b)))))
 
+;; Exercise 2.8
+(defun sub-interval (a b)
+  (make-interval (- (lower-bound b) (lower-bound a))
+		 (- (upper-bound b) (upper-bound a))))
+