In cost theory, the usual shape of the ________ curve is a rectangular hyperbola.

The correct option is average fixed cost.

Step-by-step explanation:

1. Definition of Average Fixed Cost (AFC):
Average Fixed Cost is defined as the total fixed cost (TFC) divided by the total output produced ($Q$).

$\text{AFC}=\frac{\text{TFC}}{Q}$

2. Behavior of Fixed Costs:
Total Fixed Cost (TFC) remains constant regardless of the level of output. As the volume of production ($Q$) increases, the fixed cost is spread over a larger number of units, causing AFC to continuously decrease.

3. Geometric Shape (Rectangular Hyperbola):
Since $\text{TFC}=\text{AFC}\times Q$ remains constant, the area of any rectangle formed under the AFC curve (which equals $\text{AFC}\times Q$) is always equal to TFC. A curve having the property that the area of all rectangles under it remains constant is geometrically termed a rectangular hyperbola.

4. Asymptotic Nature:
The AFC curve approaches both the horizontal (output) axis and the vertical (cost) axis, but it never touches either axis because TFC is greater than zero, and output is non-zero in production.