We have a tag transcendental-numbers.
This doesn't appear really to add anything to these challenges, 1 is also tagged math and number and the other is tagged calculus. Can we burninate it?
Optionally, we can create an approximation tag for challenges that involve approximating constants. (not sure if this is a good idea, might be too uncommon)
I've gone ahead and created a tag (and tag wiki) for approximation. I also retagged the 2 current [transcendental-numbers] questions with approximation (and removed the old tag). I'll hold off on retagging the questions listed in @Flip's answer for a bit.