The demand function shows in equation form the relationship between the units sales of good or service and one or more economic variables.

{a} The demand curve depict the relationship between quantity and price. A change in price is represented by the movement along the demand curve. A change in any other economic variable shift demand curve

{b} A pair of goods are substitutes if an increase in demand for one causes a fall in demand for other. In particular, a price cut for one good reduces sales of the other.

{c} A pair of goods are compliment if an increase in demand for one causes an increase in demand for the other. In particular, a price cut for one good increases sales of the other.

d. A good is normal if the sales increase with increase in income