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