when two goods are complements their cross-price elasticity of demand is
Cross elasticity of demand In economics, the cross elasticity of demand or cross-price elasticity of demand measures the responsiveness of the quantity demanded for a good to a change in the price of another good, ceteris paribus. It is measured as the percentage change in quantity demanded for the first good that occurs in response to a percentage change in price of the second good. For example, if, in response to a 10% increase in the price of fuel, the demand for new cars that are fuel inefficient decreased by 20%, the cross elasticity of demand would be:cross body    − 20 %  10 %  = − 2  {\displaystyle {\frac {-20\%}{10\%}}=-2} . A negative cross elasticity denotes two products that are complements, while a positive cross elasticity denotes two substitute products. Assume products A and B are complements, meaning that an increase in the price for A accompanies a decrease in the quantity demanded for B. Therefore, 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. The exact opposite reasoning holds for substitutes.
Competition (economics) Later microeconomic theory distinguished between perfect competition and imperfect competition, concluding that perfect competition is Pareto efficient while imperfect competition is not.[citation needed] Competition, according to the theory, causes commercial firms to develop new products, services and technologies, which would give consumers greater selection and better products. The greater selection typically causes lower prices for the products, compared to what the price would be if there was no competition (monopoly) or little competition (oligopoly).