problem 3 in section 4.2

[rhigginbottom]

I've tried several versions of this problem, and my answer isn't being accepted as correct on any of the three parts of the problem. It's possible that I've made a mistake! However, I have taken into account the mm to cm trick that's in the problem.

[sean-fitzpatrick]

OK, after 10 minutes staring at the WeBWorK code, I think I figured it out.
(I think these were done by @Alex-Jordan and there's a map function that I don't fully understand.)

In the volume formula, we need to divide by 10 to account for the change in units. But when we take the derivative of r^2, we get a 2, and 2/10 simplifies to 1/5. But the formula used for answer checking has a 5 in the numerator, but still has a 2 in the denominator.

Can you check to see if dividing your answer by 2 gets marked right? If so I can commit a fix tomorrow.

[Alex-Jordan]

It looks like that 2 should not be there. I set up the generic solution and it brought me to a formula with a 5 in the numerator, and no 2 in the denominator.

I'm fine-tuning a modern code block for this one and will post here soon.

[sean-fitzpatrick]

Thanks Alex.

I've got work in progress to replace NchooseK with random_subset, since the former is deprecated, and I find the latter easier to understand.

I've fixed a few coding errors in the related rates and optimization sections recently.

The perks of assigning problems from the book instead of using my thoroughly tested versions on WeBWorK, I guess!

[Alex-Jordan]

You could also take your local, vetted exercises, and use them directly in APEX now. I would not be offended! You have to xi:include them.

[sean-fitzpatrick]

I think in the long run it's better to use the problems we have now, and fix them. This might be the first time we've had classes using the problems from the book for homework, so errors are popping up now that go unnoticed when students are just using them for practice.

I'm about to make a PR that fixes this, along with a big commit to replace the NchooseK macro. If you have time to look it over, please do. If not, I've rebuilt without errors, and the questions I tested still work.

[rhigginbottom]

I'm sorry for my delay. Replying to @sean-fitzpatrick's first comment (and verifying what @Alex-Jordan has already written), dividing my answers by 2 seems to work. Thanks to you both!

[Alex-Jordan]

For Sean and anyone interested, in trying to write a modern code block for this exercise, an issue with fractions and units was discovered that we are working to fix. So I do hope to offer an example of how to do this better with PG 2.20, I'll wait until that fix is in. It would go out as hotfix for 2.20.

[sean-fitzpatrick]

Alex, is this related to the problem I ran into and reported here?

This is the problem in 4.3 about the two adjacent pens. The old code didn't work with 2.20 since the answers involve fractions with units.

I was able to get it to work with the changes suggested by Glenn.

[Alex-Jordan]

I don't think so. That issue was about code that previously worked, and stopped working with the upgrade to 2.20. The issue I am referring to is an issue that has to do only with new tools that came into play with 2.20.

[sean-fitzpatrick]

OK. I think current code is working. I'm going to merge the current changes soon (by noon, probably), because I think there are a few classes (including my own) doing these problems on Runestone right now, and we want to get them working.