## WeBWorK Problems

### Not Correctly Checking the Function ### Re: Not Correctly Checking the Function

by Davide Cervone -
Number of replies: 0
Can you give the answer the student actually entered? I did the simplification and WW marked it correct.

For me, with seed 188 I get the original question being [dmath]g(t) = \frac{9}{\sqrt{t^{8}}}+2\sqrt{t^{2}}-8t^{10}[/dmath] so I simplified as [dmath]g(t) = \frac{1}{t^4}+2|t|-8t^{10}[/dmath] which gives a derivative of [dmath]g'(t) = -\frac{4}{t^5}+2\frac{t}{|t|}-80t^9[/dmath] which WeBWorK marks as correct.

Could it be that the student "simplified" $\sqrt{t^2}$ as $t$ rather than $|t|$? Since the values of $t$ range over both positives and negatives, that would make a difference. Of course, the derivative of $|t|$ is the signum function [dmath]\mathop{\rm sgn}(x)=\begin{cases}-1&{\rm if\ }x < 0,\cr 1&{\rm if\ }x &gt 0,\cr {\rm undefined}&{\rm if\ }x=0\end{cases}[/dmath] which I obtained by using $t/|t|$.

As for seeing the test points, you can do that by adding diagnostics=>1 to the answer checker:

    ANS(\$ans->cmp(diagnostics=>1));


This will produce a table of the values, color-coded to see which ones are marked correct and which aren't, and when the function is a single-variable function, it will show the graph of the function and the errors.

Hope that helps.

Davide