A={1,2},B={2,1}

The sets \( A = \{1, 2\} \) and \( B = \{2, 1\} \) are considered equal in set theory, even though they are written in a different order. In set theory, the order of elements in a set does not matter, and duplicate elements are not allowed. Thus:

\[ A = B \]

If you have further questions regarding sets or any other topics, feel free to ask!