In each case marginal value indicates the rate of change. With a small variation in the quantity of x, the marginal utility changes at the rate of 2, or with a small change in the quantity of x, marginal revenue changes at the rate of 0.75. In both these examples , the sign of the marginal values is positive. Therefore both marginal utility and marginal revenue tend to increase with every increase in the quantity of x. This however need not always be the case. In the present case, the functions are said to be rising. But there may be falling functions as well, such as that of the cost of production in the initial stages. In that case, value of the marginal cost may be negative. In fact, productive activity normally and beneficially occurs on the falling phase of the average cost curve . This will get clearer as we proceed.
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