Determine where the function is concave downward. 
Definitions:
Cross-Price Elasticity
A measure of how the quantity demanded of one good changes in response to a price change of another good.
Complementary Goods
Products or services that tend to be used together, where the consumption of one enhances the use of the other.
Income Elasticity
A measure of how much the demand for a good changes in response to a change in consumers' income.
Cross-Price Elasticity
A measurement of how the quantity demanded of one good responds to a change in the price of another good.
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