The first assumption of law of demand is that the tastes and preferences of the consumer are same regardless of the income group. The second assumption is that all consumers have a fixed income and there is no change in income over a period of time. Thirdly, the prices of the related goods do not change and they are fixed. Moreover, all other economic factors are constant and they are ignored. On the basis of all these assumptions the demand curve is derived which depicts that prices are inversely proportional to quantities that is when price decreases the quantity demanded increases.
