explain xed value

uni/mmme/3049_engineering_management_2/xed.md

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 uuid: d6b6e3dd-1bba-466a-aad8-9e39c68280ab
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+# Cross Elastic Demand (XED)
+
 The equation given in the lecture slides is:
 
 $$\text{XED} = \frac{\frac{\Delta q_A}{q_A}}{\frac{\Delta p_B}{p_B}}$$
@@ -17,6 +19,9 @@ $$\text{XED}
 = \frac{\text{percentage change in quantity of A}}{\text{percentage change in price of B}}
 $$
 
+If XED is positive, the two goods A and B are substitutes for each other.
+If XED is negative, the two goods are complimentary.
+
 ## Example
 
 ![A question from the book (page 91)](./images/xed_question.png)