Shouldn't the x/2 at 7:52 be positive or negative as well?

This post is deleted! 
[Originally posted in the Discussions]
Shouldn't the x/2 at 7:52 be positive or negative as well?

In this case, since x represents the length of the diagonal of the trapezoid, it wouldn't be possible for x/2 to be a negative number. In general, though, you are right; this equation falls in to the category of "quadratic equations," which have an xsquared term. It's possible to show that quadratic equations always have two solutions.
You can see Prof. Loh talk about a cool way of solving quadratic equations which he came up with while developing Module 4: Algebra Tools. You can also read more about him and his quadratic method in this NYT article.
Happy Learning!
The Daily Challenge Team