### cross-price elasticity of demand

the percentage change in the quantity demanded for one good divided by the percentage change in the price of another; This value is greater than zero for substitutes and less than zero for complements

$E_{ab}=\frac{\%\Delta Q_{Da}}{\%\Delta P_b}$

### elastic demand

demand in which the percentage of the quantity demanded changes more than the percentage change in price

### elastic supply

supply in which the percentage change in quantity supplied is greater than the percentage change in price

### elasticity

a measure of how responsive one variable is to changes in another

### income elasticity of demand

the responsiveness of demand to changes in consumers' income, as measured by the percentage change in the quantity demanded divided by the percentage change in consumers' income; this value is greater than zero for normal goods and less than zero for inferior goods

$E_Y=\frac{\%\Delta Q_D}{\%\Delta Y}$

### inelastic demand

demand in which the quantity of a good or service that consumers demand is relatively insensitive to changes in price; the percentage change in quantity demanded is smaller than the percentage change in price

### inelastic supply

a situation in which the quantity of a good or service that producers supply is relatively insensitive to changes in price; the percentage change in quantity supplied is smaller than the percentage change in price

### midpoint formula

formula used to calculate the percentage change in a variable by dividing the change in the variable by the average of the the initial and final values

$\frac{B_2 - B_1}{(B_2 + B_1) / 2} ÷ \frac{A_2 - A_1}{(A_2 + A_1) / 2}$

### perfectly elastic demand

a situation in which consumers respond to any price increase by no longer purchasing the good; the quantity demanded falls to zero, the demand curve is horizontal, and the value is infinite

### perfectly elastic supply

a situation in which any change in price causes the quantity supplied to fall to zero; when the price elasticity of supply is infinite

### perfectly inelastic demand

a situation in which the quantity demanded does not change in response to a change in price; the percentage change in quantity demanded is equal to zero, no matter how price changes, the demand curve for the good is vertical, and the value is equal to zero

### perfectly inelastic supply

a situation in which any change in price of a good or service leads to no change in the quantity supplied; the supply curve is vertical and the price elasticity of supply is equal to zero

### price elasticity of demand

a measure of how responsive the quantity demanded of a good is to changes in its price

$E_D=\frac{\%\mathrm\Delta Q_D}{\%\mathrm\Delta P}$

### price elasticity of supply

a measure of how responsive the quantity supplied of a good is to changes in its price

$E_S=\frac{\%\mathrm\Delta Q_S}{\%\mathrm\Delta P}$

### total revenue

the total amount earned by selling a good or service; the price of a good or service multiplied by the quantity sold

$TR = P \times Q$

### unit elastic demand

a situation in which the percentage change in demand is equal to the percentage change in price; when the price elasticity of demand is equal to 1