Wiktionary
exercise for the reader
n. 1 (&lit exercise for the reader English) 2 (label en by extension) (l en A matter left to the addressee's judgement to decide.)
Usage examples of "exercise for the reader".
To borrow the phrase so beloved of textbook writers, it is left as an exercise for the reader to judge if such methods could have given results justifiably acclaimed as being highly significant.
The answer, of course, is best left as an exercise for the reader.
Applying this to the case of Apple Computer will be left as an exercise for the reader, and not a very difficult exercise.
They will be left, as the mathematics text-books put it, as an exercise for the reader.