I had an electrodynamics textbook (Griffiths) that was the worst since it did this all the time.
It made studying it really hard because it skipped so many intermediate steps that I couldn't follow it and ended up even more confused.
It was one of the few courses I ended up with a C in, despite trying really hard at it.
It's one of the reasons I ended up in engineering rather than physics or mathematics. The numbers are more intuitive and everything isn't an obscure proof.
Maths*
That's when you ask for their proof.
The proof is left to the reader
the fun thing about this is that the textbooks get more judgy the more advanced the material gets. heres an example from a graduate level algebra textbook by lang.
diffemples
If it's obvious, then you didn't need to say it.
I.E. "I don't feel like writing out the proof." See also: "this is left as an exercise for the reader."
I just absolutely love the transition from the bottom left bit to the next one. Diffemples.
