Answer:
Opportunity costs are defined as the additional costs or benefits lost from choosing one activity or investment over another alternative. It is a relative concept because you cannot be 100% sure that the other investments or activities would have yielded a specific gain.
For example, when you calculate the economic cost of starting your own business, you consider your current salary as an opportunity cost. But what happens if you get fired (or the company closes), your opportunity cost would have been $0? Or how can you exactly measure your future salaries? Maybe in a couple of years you get promoted to manager, or maybe not?
The same applies to economies, since the opportunity cost of producing certain tradable goods is not always fixed, it might decrease or increase due to productivity or efficiency changes. But in order to calculate or determine we must include the most probable option.
In microeconomics, a strictly convex production possibilities frontier function must include a combination of both goods. In strict convexity, the second derivative f''(x) ˃ 0, so the PFF curve cannot be straight, it must have a slope.
When we calculate the opportunity costs of PPF, we usually try to determine which product has the lowest opportunity cost, but that is not an interior solution because both goods are not being produced (the curve is not strictly convex). On a strictly convex curve, as you approach the extremes the opportunity cost of producing one good is high, but on the center the opportunity cost is much lower.
