The graph to the right illustrates the demand and marginal revenue curves for a monopolist who has experienced an increase in demand for its product. Notice that the marginal revenue curve shifts up just like the demand curve. (As always, there is no reason to suppose that such shifts are actually parallel or that the curves need to be straight lines).
Marginal revenue must change with demand since it represents the change in revenue
due to an increase in sales. Since revenue and price are both
dependent on market demand, so is marginal revenue.
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