I looked at problems from that set. They use \( \ \) to generate a
space. When dvipng is asked to turn that into an image, it gets
confused by the fact that the image is blank. Actually, I haven't
tracked it down to the source -- the bug may be in gd, the underlying
graphics library which makes the png images.
I will see if I can get the dvipng maintainer to look at fixing this.
In the meantime, it seems that this is not the best way to handle the
original problem. That is, whoever wrote the problem wanted to insert a
hard space, presumably for aesthetic reasons. Currently, this would be
accomplished on the web page by creating a blank image and displaying
it to create space. But, some browsers don't support transparent
backgrounds in png's, so the result would not be particularly aesthetic
after all. On the other hand, both latex and html have ways of creating
hard spaces. A better approach would be to use $SPACE, a WeBWorK macro
for doing what is desired, and which changes its output to do the right
thing in each mode.
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