平均固定コスト、平均変数コスト、平均合計コスト II

The average fixed cost curve slopes downward because fixed costs remain constant regardless of output. As production increases, the same total fixed cost is divided across more units, reducing the cost per unit. This relationship causes the AFC curve to decline continuously and approach zero without ever reaching it.

The average variable cost curve is U-shaped due to changing returns to scale. At low output levels, firms experience increasing returns, lowering average variable costs. Eventually, diminishing returns set in, causing average variable costs to rise. This transition from increasing to diminishing returns creates the characteristic U shape.

Average total cost is the sum of average fixed cost and average variable cost. Since AFC declines continuously while AVC initially decreases then increases, the ATC curve remains above the AVC curve. The gap between them narrows as output increases because fixed costs spread across more units.

Average total cost eventually rises because increasing average variable costs eventually outweigh the decreasing average fixed cost. Initially, the declining AFC dominates, keeping ATC falling. However, as diminishing returns intensify, rising AVC eventually exceeds the rate of AFC decline, causing ATC to increase and form its characteristic U shape.

The average total cost curve begins rising after the average variable cost curve reaches its minimum point. At this inflection point, the rate of increase in average variable costs surpasses the rate of decrease in average fixed costs. Beyond this point, the upward pressure from AVC dominates, causing ATC to increase.

The gap between average total cost and average variable cost curves decreases as output increases. This occurs because average fixed cost continuously declines with higher output levels. Since ATC equals AVC plus AFC, the shrinking AFC reduces the vertical distance between the two curves.

Understanding how average costs behave requires examining their relationship with marginal costs. When marginal cost falls below average cost, it pulls the average down. When marginal cost rises above average cost, it pulls the average up. This dynamic determines whether average costs are declining or increasing at any output level.