The question involves a decision about future production. Therefore, it is a long run phenomenon that involves long run marginal costs and long run average costs When we connect these two costs measures their behavior are closely related At each output level LAC is simply total cost divided by that output level. However, marginal costs are incurred by moving from.one level to another.
Two facts arise here, when LMC is less than LAC, LAC is falling and rising when LMC is greater than LAC. LAC is at minimum at the output level at which LAC and LMC cross.. It holds that when the marginal cost of the next unit exceeds the average cost of the existing units, making the next unit must drag up average cost. Conversely, when the marginal cost of the next unit luez below the average cost of existing units, an extra unit of production drags down average costs.