Unless you continue to study mathematics, which you will likely only do if you stay in academia, you will forget what you've learned! You've remembered more th

Unless you continue to study mathematics, which you will likely only do if you stay in academia, you will forget what you've learned! You've remembered more than you think, most notably you do (or would if appropriately prompted) remember that things are possible and probably a bit about the approach. You don't remember the details of how, but those are details you can look up if needed - you have the framework in which those details fit and that's the important thing.