By adding all the marginal products at each level of employment of the variable input, we obtain a value equal to

The value obtained by adding all the marginal products at each level of employment of the variable input is equal to the total product. The total product is the total output produced by a firm using a given combination of inputs, including both fixed and variable inputs. The marginal product, on the other hand, is the additional output that is produced by using one more unit of the variable input, while holding all other inputs constant. By adding up all the marginal products, we can calculate the total amount of output that is produced using all units of the variable input. For example, if a firm produces 10 units of output using 2 units of labor and 1 unit of capital, and it produces 20 units of output using 4 units of labor and 1 unit of capital, then the marginal product of labor is 10 units of output (20 - 10), which is the additional output produced by using 2 more units of labor while holding the amount of capital constant. If we add up all the marginal products of labor at each level of employment of labor, we obtain the total product, which is the total output produced using all units of labor. In summary, adding all the marginal products at each level of employment of the variable input gives us the total product, which is the total amount of output produced by a firm using a given combination of inputs.