Introduction
Let us first understand the concept of elastic, inelastic and unitary elasticity of demand.
Elastic demand - Horizontal straight line to y-axis, parallel to x-axis, is known as perfectly elastic demand curve. It shows that 
. It is a situation when even a slightest rise in price leads to zero demand for the commodity.
In other words, % change in Qd is greater than % change in P.
Inelastic demand - Vertical straight line, parallel to y-axis, is know as perfectly inelastic demand. It shows that 
. It is a situation when change in price causes no change in the quantity demanded.
In other words, % change in Qd is less than % change in P.
Unitary demand - A rectangular hyperbola is a curve under which all rectangular areas are equal. It shows that 
. It is a situation where total expenditure remain constant in response to increase or decrease in price of commodity.
In other words, % change in Qd is equal to % change in P.