If I use Matrix([[1/2,1],[1,1]]), then the correct answer is displayed as a matrix but the fraction becomes 0.5.
How can I keep both the fraction and the matrix form in the correct answer field?
singleResult=>1is used with the MultiAnswer object. That is important information that we would have had if you had provided a full example.
singleResult => 0, it will format as you want it to, but the single-result version doesn't generate the required TeX form.
parserMultiAnswer.plthat should take care of the problem.