So, you'll memorize them.

A few that you need, on an ad-hoc basis, instead of front-loading a bunch of calculations you're probably never going to use.

Memorizing every combo of 2-9 x 2-9 is only 36 numbers. That doesn't seem like a big waste of time to me.

If you do common ones like 2, 3, 5, and squares, that already covers most of them and you only have 10 left to remember them all.