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.
