Error multiplying functions

[Martin Jones‏]

I have been using the expression command to create randomised algebraic expressions. But it doesn't behave correctly in the way I expect expressions to work.

For example f is defined expression(2+"t+"+5) then it shows as 2t+5 as expected.

But if I try to evaluate t*{f} or t*({f}) then the answer is 2t^2+5 instead of 2t^2+5t.

Example question: https://numbas.mathcentre.ac.uk/question/142459/error-multiplying-functions

Thanks.

[Timur Zaripov‏]

Dear Martin,

Not sure is by design, but what happens is more like a string substitution, where {f} is replaced by expression f, rather then algebraic substitutions, where {f} is replaced by expression (f).

As a workaround you could use t*({f}) which will do what you expect.

With best regards,

Timur Zaripov.

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[Martin Jones‏]

Hi Timur,

I did try using t*({f}) and the result is the same.

Martin

[Timur Zaripov‏]

Dear Martin,

I get the result shown in screenshot below:

I have given you access to my test question to see if we do anything differently.

With best regards,

Timur Zaripov.

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[Christian Lawson-Perfect‏]

I think this is a bug, sorry! I'll see try to fix it now.

Timur: Martin put "t*({f})" in the expected answer field for a mathematical expression part. Fortunately, it looks like your method of defining another variable would do as a work-around for now.

And I need to find out why there seem to be two different methods of substituting into expressions!

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[Martin Jones‏]

Christian,

We have a few tests in https://numbas.mathcentre.ac.uk/question/142579/substitution-test/ some of which work and some don't. May help with your diagnosis.

Martin

[Christian Lawson-Perfect‏]

I think I've now fixed this. Thanks for the useful test cases!

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[Martin Jones‏]

Thanks, that is indeed looking better.

There is just perhaps the puzzling preview that appears in the marking settings:

[Christian Lawson-Perfect‏]

Can you try reloading the page? I think MathJax has failed to load for you. It looks OK on my PC.

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