Marginal Cost and Marginal Revenue

Marginal cost and marginal revenue are just the rate-of-change idea from the previous section applied to a firm's cost and revenue functions. Rather than asking how much it costs (or earns) to produce $x$ units in total, marginal cost and marginal revenue ask how fast that total cost or revenue is changing at a given output level $x$ — informally, close to the cost or revenue of producing one additional unit.