What's your real life experience of "solved it is, trivial it is"?

That is always how math works.

At the beginning of the semester, I expect the students in my elementary algebra class to struggle to solve an equation like "3x+4=12."  It is new and challenging to them.

By the end of the semester, we are solving quadratic equations.  We get to the point where we have exactly that problem that they struggled with three months ago.  I don't need to spend time on it any more -- they have struggled with the concept enough that they have it down now.  So, I say, "You know how to do these now," and jump to the solution.

I have a limited amount of contact time with my students.  I need to make sure that I am spending that time on concepts where the majority of my students need help, and not spending time on concepts where the majority of my students already know the process.  Even if they cannot do the problems as fast as I can.  Yet.

If you are not experiencing this constantly, then you are either in the wrong class or just not paying attention.
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My analysis prof from the first semester used to say that you can call a fact trivial if and only if you can prove it. I think it is a good mindset
Rather than calling things trivial or easy, I usually just say something along the lines of “this is analogous to existing results” or “one can show through direct calculation”
Sometimes someone will say that a little thought will verify some claim or other, but I have trouble coming up with the specific little thought.
I hate it and all synonimous expressions if used in a book. The reason is simple, it's impossible to know the knowledge of your audience. You might assume some things vut you cannot know it. It does not bring anyone further, just explain it you bothered to write the book already why not make it as accessible as possible.

In lectures it heavily depends on how advanced it is, how good the teacher/prof etc knows their audience and much intersections the topic has with other fields. Small sidestory; i took funtctional analysis last winter, we talked about spectral mapping theorems etc and went on to distributions. To do that you need to have a decent knowledge in topology. Nothing fancy but you need to know a few ins and outs. But where i study topology is not mandatory, which is a mess i know especially when intro to numerical analysis is. So we talked about topology a little and did some excersises. In that setting, i would not recommend calling anything trivial except you can be sure it has been topic in a very basic mandatory lecture. In general, i would avoid it.

I feel like in most cases i just makes the people it is not obvious too feel dumb and to afraid to ask since they do not want to give away that impression.

If you did something similiar in the very recent past, you can just shorten it and mention when you did something special.

Articles are kind of unique since they are supposed to be compact/reasonable short and target a very educated audience by design. So the bar to understand it lies higher naturally.
My most vivid memory of such act is proof by induction. Especially the d^n (f(t)) /dt^n problems . When I first encountered such problems I was extremely confused . I know always remember that incident and keep laughing about how such trivial topics were so hard . Now whenever I encounter any challenging topics I remind myself with such memory to encourage myself to dwell deeper into the topic without getting intimidated
As a physicist, working in my master's thesis I once spent over a week in a derivation that required to handle and shift around a series expansion. In the thesis I provided no proof for it and just stated "after gathering odd and even terms separately, it can be easily verified that ..."
I'm currently writing a rather comprehensive (pre-edited, if published in its current form: roughly 2800 textbook-sized pages) book on STEM.

One of the reasons it's currently so massive is that I'm trying really hard to not assume things like this, for the reader's sake.
Galois correspondence. Once you get it it's really hard not to think "isn't this all just trivial?"

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