Khan/khan-exercises — pull request #17826 by csilvers (2012-03-28 20:18:39+00:00)
Title: When asking people to determine the slope, do not show lines with slope +/-1

This is based on feedback from a user:

> From my tutoring experience I have a suggestion for the problems
> associated with 'Slope of a Line'. One class of problems gives two
> points and asks the user to find the slope of the line. My suggestion
> is not to include problems where the slope is 1, or -1. A common
> confusion in the youth is if change in y or change in x is in the
> numerator. In problems where the slope is 1, or -1 you can mix up the
> dx and dy and still get the problem right leading to more confusion.

Slope 1/-1 is rather uncommon in this exercise anyway, so it's not a
big change to disallow it.