  Comparison of answers offered by a computer algebra system (CAS) with answers
derived by a student without a CAS is relevant, for instance, in the context of
computer-aided assessment (CAA). The issues of identity, equivalence and
correctness emerge in different ways and are important for CAA. These issues
are also interesting if a student is charged with the task of comparing the
answers. What will happen when students themselves are encouraged to analyse
differences, equivalence and correctness of their own answers and CAS answers?
What differences do they notice foremost? Would they recognise
equivalence/non-equivalence? How do they explain equivalence/non-equivalence?
The paper discusses these questions on the basis of lessons where the students
solved trigonometric equations. Ten equations were chosen with the aim to
ensure that the expected school answer and the CAS answer would differ in
various ways. Three of them are discussed more thoroughly in this paper.
