u/Cffex

My initial assumption was finding the interval on which its first derivative is positive, which yields R \ [-1,1].

However, the answer provided by my teacher was R \ (-1, 1).

I asked why the 1s imply increasing, and he said that the question is badly worded, and my solution was correct and his was wrong. He explained why but I did not understand.

I asked AI and it explained that his solution was correct due to the way increasing on a set is defined.

I tried searching online but found no results, except from MathWay but for some reason I was blocked from accessing the site.

So what's the answer?