Khan/khan-exercises — pull request #75721 by chris-torrence (2013-09-15 06:05:05+00:00)
Title: Improve hints for rational expressions

Several recent issues have reported bugs with this exercise: #75297,
#71661, #71595. Improve the hints for the 3 problems with rational
expressions, to make it clear that this isn't a "bug" but is just the
definition of the domain of a function.