The demand function shows in equation form the relationship

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