Answer:
Step-by-step explanation:
Recall that, in this case, the subset of X for which R is defined is called the domain of R. The mistake occurs when we assume that the domain R is the whole set X, but it could happen that R is not defined for some elements of X.
Recall the following example:
X = {2,4,6}.
We can define R as follows {(2,2), (4,4), (2,4), (4,2)}. We can easily check that this is a transitive and symmetric relation, but since we don't have the element (6,6) it fails to be reflexive.
