Been doing some more revision sessions on CIE AS economics and went through how the elasticity of demand varies along a demand curve. Notice in Case A that the fall in price from Pa to Pb causes the the total revenue to increase therefore it is elastic – the blue area (-) is less than the orange area (+). In Case B the opposite applies – as the price decreases from Pa to Pb the total revenue decreases therefore it is inelastic – the blue area (-) is greater than the orange area (+). In Case C the drop in price causes the same proportionate change in quantity demanded, therefore there is no change in total revenue – it is unitary elasticity.
Remember where MR = 0 – PED = 1 on the demand curve (AR curve). A particularly popular question at A2 level is ‘where on the demand curve will a profit maximising firm produce at?’. As MC = MR above zero the imperfect firm always produces on the elastic part of the demand curve.