Final answer:
The equation that relates the quantities is y = (1/2)dz, where y varies jointly with d and z, and the constant of proportionality is 1/2.
Step-by-step explanation:
When a quantity varies jointly with two other quantities, it means the quantity is directly proportional to the product of the other two quantities. Given that the constant of proportionality is 1/2, if we let y be the quantity that varies jointly with d and z, we can write the equation as y = (1/2)dz.
This equation tells us that as d and z increase, y will also increase, and conversely, as d and z decrease, y will decrease, both maintaining a consistent ratio determined by the constant of proportionality, which in this case is 1/2. For mathematical convenience, this equation may be rearranged to express d or z in terms of the other variables if needed.