If marginal costs are relevant for specific situation or specific decision making scenario then marginal costs are relevant costs otherwise marginal costs can be irrelevant.

One is able to learn about marginal costs at several different places online, such as at the following websites: the Wikipedia Marginal Costs webpage, Marginal Cost, and Margins.

Profits will be maximized when marginal revenue is equal to marginal costs. This will only happen in cases where there are fixed costs.

Rational Decision making occurs when marginal benefits of an action exceed the marginal costs

equal to marginal revenue