I've seen tons of these used by university lecturers.
I have had a few math books use something similar, I believe a differential equations book and a linear algebra book with many 'We leave this to the reader to prove'. Which is fine if you know the subject but when you are learning it is by far one of the most annoying things I have come across.
I've used some of these - and received part marks for them - when I got stuck on tests or exams.