If products A and B are complements, an increase in the price of B leads to a decrease in the quantity demanded for A, as A is used in conjunction with B. Equivalently, if the price of product B decreases, the demand curve for product A shifts to the right reflecting an increase in A's demand, resulting in a negative value for the cross elasticity of demand. If A and B are substitutes, an increase in the price of B will increase the market demand for A, as customers would easily replace B with A, like McDonald's and Domino's Pizza.

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