## WeBWorK Problems

by Peter Selinger -
Number of replies: 1
I am looking at this problem:

Library/Union/setMVlinesplanes/an12_6_15.pg

It generates implicit equations for a few planes, using ImplicitPlane($P,$N) to construct them.

For some reason, the planes are typeset like this:

8y−2x+6z=36

9x+12y−13z=−15

2y−2x+3z=−5

27z−(17x+4y)=19.

Note how:

* the variables appear in some random order,

* sometimes the variables are parenthesized in a weird way, for example 27z−(17x+4y)=19.

I find this behavior undesirable because it makes the question seem like a "trick question": most of the time we write variables in alphabetical or some other predetermined order, and writing them in random order is a gratuitous complication (unless the very point of the question is to standardize a formula, which in this case it is not).

My question: how can I turn this behavior off, so that the implicit planes are typeset in standard format, i.e.

-2x+8y+6z=36

9x+12y-13x=-15

-2x+2y+3z=5

-17x-4y+27z=19.

The strange thing is that I see nothing in the problem definition that randomizes the variables. All planes are uniformly generated. I tried altering the setting

Context()->reduction->set('(-x)-y'=>0);

but it doesn't make a difference. The documentation of ImplicitPlane here

http://webwork.maa.org/wiki/ImplicitPlane

does not mention that the order of variables will be randomized. The documentation here

http://webwork.maa.org/pod/pg_TRUNK/macros/parserImplicitPlane.pl.html

says that we can specify the order, for example ImplicitPlane($P,$N, ['x','y','z']). However, when I tried this, the output didn't change at all; it was the same as before.

Is there some setting that I am missing? Thanks, -- Peter

by Peter Selinger -
Upon further investigation, I answered my own question as follows: in the problem in question, we have

Context("Vector")->variables->are(t=>'Real');
Context()->reduction->set('(-x)-y'=>0);

and the further down in the same problem:

Context("ImplicitPlane");

So any updates done to the "Vector" context (including the reduction->set in the original problem text) apparently have no effect for the later typesetting of the implicit planes. However, if I put

Context()->noreduce("(-x)-y","(-x)+y");

*after* the Context("ImplicitPlane"), then the undesirable formatting can indeed be turned off. The reason my earlier efforts had no effect is that I put the "noreduce" command after Context("Vector") near the top of the problem.

For this problem, I suggest adding

Context("ImplicitPlane");
Context()->noreduce("(-x)-y","(-x)+y");

Thanks, -- Peter