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I received the error message "Question Syntax. There is an error in the syntax of this question. One or more variables may be invalid." when I tried to make the following question:

Compute $\|\langle [x], [y], [z] \rangle \times \langle [u], [v], [w] \rangle \|$.

The answer I entered was

$\sqrt{(y \times w - z \times v)^2 + (z \times u - x \times w)^2 + (x \times v - y \times u)^2}$.

Can anyone help me, please ?

EDIT: I am using the "blackboard learn" course management software. I am a teacher, making an online quiz for my students.

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    $\begingroup$ Blackboard is a course management tool used in many Universities: en.m.wikipedia.org/wiki/Blackboard_Learn. Those who are unfamiliar with it will of course be unable to answer my question. I had hoped that some users of this forum might have experience with it. Although you don't know it, in case there might be some users who do know it, I would ask that you please not vote to close my question. I am indeed a teacher trying to create a question, as I hoped I had conveyed by writing "...when I tried to make the following question...". Apologies if I was not clear. $\endgroup$ – Simon Nov 13 '16 at 14:07
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    $\begingroup$ I apologize that I misunderstood you. I've retracted my close vote. I suggest that you edit your post to include the details you've put as comments. $\endgroup$ – Joel Reyes Noche Nov 13 '16 at 14:23
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    $\begingroup$ It's really disheartening to read the comments and realize the lack of reading and understanding skill,as well as knowledge, people commenting have. The notation used in the text question is irrelevant for blackboard. What blackboard asks is that you define variables in the text so that they can be substituted by values defined by the program when the student runs the quiz. It seems that blackboard programmers were unable to follow their own conventions... [mod note: see below for continuation] $\endgroup$ – Dr. R Jan 25 '17 at 19:26
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    $\begingroup$ Just to make sure I am understanding the question: is the X symbol you are using to denote multiplication of scalars the symbol that Blackboard wants you to use, or just one you are choosing here for clarity of presentation? $\endgroup$ – mweiss Jan 25 '17 at 19:57
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    $\begingroup$ Dr. R, Thank you very much for your very reassuring comments. The feedback was even harsher than you see here, before some posts were retracted by their posters ! I suspect that Socrates, were he a Stack exchange member, would have the lowest reputation score among all members... In the end I gave up on the blackboard automated quizzes and used instead those of the publisher of the textbook, namely McGraw-Hill. These worked excellently. $\endgroup$ – Simon Jan 25 '17 at 21:45
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The problem is that the question was written using the Blackboard graphical formula editor, which cannot recognize the square brackets as variable definitions. For that you need to put the square-bracket variables in normal Blackboard text, not the formula editor. (Note the UI directions on the top of the screen: "Add question text that includes variables in square brackets [x] corresponding to variables in the formula that will be used to calculate the answer to this question.").

Having just tested it in Blackboard: The answer formula is fine. If I write the question using a Blackboard formula, then I get the same error as you. But if I instead write the question using basic text (like "Compute ||<[x], [y], [z]> x <[u], [v], [w]>||"), then that is accepted without the error. Obviously that text version isn't perfectly formatted, so you might need to play around with fonts or special characters to get it looking right.

Alternatively, you could write the question in natural language, as I think that feature is intended, i.e., "Find the norm when multiplying vectors <[x], [y], [z]> and <[u], [v], [w]>".

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