I think there was a bit of misunderstanding with the question a student asked around 17:30. The student asks why the right hand limit must exist. When the instructor wrote down "right hand limit" he didn't mean the limit from the right hand side (x->a+), he literally mean the limit he wrote on the right.Technically, the right hand limit (x->a+) has to exist otherwise the full limit wouldn't exist, but that goes without saying.####[]