Now let's discuss how another related good, a complementary good, can affect the demand for a product. Just like with substitute goods, complementary goods can also impact our demand. Complementary goods are items that are usually bought together. We'll cover some examples shortly, but the main concept is that when the price of good X increases, the demand for good Y decreases. This happens because these items are typically purchased together. So, if the price of one increases, consumers may decide not to buy both items.

Unlike directly proportional relationships, we now define inversely proportional relationships. These are variables that move inversely to each other: when one goes up, the other goes down, and vice versa. With complementary goods, this is exactly what happens. The price of one good increases and the demand for the other good decreases. They have an inverse relationship, also known as an inversely proportional relationship. This scenario involves a change in price of a different product affecting the demand for the product we are analyzing.

Let's discuss some examples of complementary products. A common pair is peanut butter and jelly. If the price of peanut butter increases, the demand for jelly will decrease. This makes sense because they are generally purchased together and a price increase in peanut butter might discourage buyers from purchasing both. Similarly, consider DVD players and DVDs, which are also complementary. If the price of a DVD player decreases, the demand for DVDs will likely increase, as they are usually bought together, and a cheaper DVD player makes DVDs more appealing. Lastly, consider cars and gasoline. If the price of cars goes up, the demand for gasoline will likely decrease.

This logical approach, utilizing our intuition about how products affect each other, is something we will explore more in our upcoming lessons. Let's go ahead and do an example.