ask about elastic

你個問題有多少問題，因為elasticity 有好多種，如有：
In microeconomic we have the following:
price elasticity of demand
income elasticity of demand
cross elasticity of demand

In macroeconomic we have the following:
incom elasticity of money deamnd
interest elasticity of money demand
interest elasticity of investment

真係睇你想講邊種先.....你個問題都唔清楚叫人點答姐

ELASTICITY ALTERNATIVES:
The relative response of one variable to changes in another variable. Elasticity is commonly used in the study of market exchanges to identify the relative response of quantity (demanded and supplied) to changes in price.
Five categories of elasticity that form a continuum indicating the relative responsiveness of a change in one variable (usually quantity demanded or quantity supplied) to a change in another variable (usually price).
Alternative// Coefficient (E)
Perfectly Elastic //E = ∞
Relatively Elastic// 1 < E < ∞
Unit Elastic// E = 1
Relatively Inelastic// 0 < E < 1
Perfectly Inelastic// E = 0
The chart to the right displays the five alternatives based on the coefficient of elasticity (E). The negative value obtained when calculating the price elasticity of demand is ignored to allow for comparison with the price elasticity of supply.
Perfectly Elastic
The chart begins with perfectly elastic, given by E = ∞. Perfectly elastic means an infinitesimally small change in price results in an infinitely large change in quantity demanded or supplied. This elasticity alternative exists when the price is fixed, that is, an infinite range of quantities is associated with the same price. Perfectly elastic demand can occur, in theory, when buyers have the choice among a large number of perfect substitutes-in-consumption. In an analogous way, perfectly elastic supply can occur when producers have the ability to switch resources among a large number of perfect substitutes-in-production.
Relatively Elastic
The second category is relatively elastic, in which the coefficient of elasticity falls in the range 1 < E < ∞. That is, the coefficient is between one and infinity. With relatively elastic demand and supply, relatively small changes in price cause relatively large changes in quantity. Quantity is very responsive to price. The percentage change in quantity is greater than the percentage change in price. Relatively elastic demand occurs when buyers have the choice among a large number of close but not perfect substitutes-in-consumption. In an analogous way, relatively elastic supply occurs when producers have the ability to switch resources among a large number of close but not perfect substitutes-in-production.
Unit Elastic
The third category is unit elastic, in which the coefficient of elasticity is E = 1. In this case, any change in price is matched by an equal relative change in quantity. The percentage change in quantity is equal to the percentage change in price. Unit elastic is essentially a dividing line or boundary between elastic and inelastic.
Relatively Inelastic
The fourth category is relatively inelastic, in which the coefficient of elasticity falls in the range 0 < E < 1. That is, the coefficient is between zero and one. With relatively inelastic demand and supply, relatively large changes in price cause relatively small changes in quantity. Quantity is not very responsive to price. The percentage change in quantity is less than the percentage change in price. Relatively inelastic demand occurs when buyers can choose only among a small number of imperfect substitutes-in-consumption. In an analogous way, relatively inelastic supply occurs when producers have a limited ability to switch resources among a small number of imperfect substitutes-in-production.
Perfectly Inelastic
The final category presented in this chart is perfectly inelastic, given by E = 0. Perfectly inelastic means that quantity demanded or supplied are unaffected by any change in price. The quantity is essentially fixed. It does not matter how much price changes, quantity does not budge. Perfectly inelastic demand occurs when buyers have no choice in the consumption of a good. In an analogous way, perfectly inelastic supply occurs when producers have no ability to switch resources among the production of goods.