In order to see that the slope of the total utility curve is marginal utility it helps to look at a small portion of that curve up close, as we've done to the right. (The 'broken' section on the vertical axis indicates that part is missing to allow us to zoom in.)

    Marginal utility is the additional utility enjoyed from consuming one more unit. When our consumer increases consumption from 4 episodes to 5 her utility increases from 10 to 15, so the marginal utility of the 5th episode is 5. The slope of the TU curve between 4 and 5 units is approximately 5 as we can see to the right. Thus, the slope of the TU curve is marginal utility since the slope represents the rate of increase in utility as consumption increases.

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