0.999… = 1 in theory also.

Otherwise, there exists a δ such that 1 - δ = 0.999…

Then, the δ should have a first nonzero digit. Let us say it is in the millionth placd. But then, 0.999… cannot have a 9 in the millionth place.

It is in theoretical math as well. I just woke up and don't know the proof by heart but there is a proof for 0.99 repeating being true equal to 1.

Even in theoretical math, 0.999 repeating ends up being exactly equal to 1. In fact, any terminating decimal can be rewritten in a similar manner. For example, 0.25 is exactly equal to 0.24999999 repeating

What's 3 * 1/3? What's 3 * 0.3333333...?