[질문] x=1, y=2, ∴x y=3, ∴x y 2=3 2=5

The statements you provided are not mathematically correct.

If x = 1 and y = 2, then xy = 1 * 2 = 2, not 3.

Similarly, xy2 = 1 * 2 * 2 = 4, not 5.

Therefore, the statements x y = 3 and x y 2 = 3 2 = 5 are incorrect.