This statement is not true in general.
Two sets A and B are equivalent if they have the same number of elements, i.e., if |A| = |B|.
The given equation n(A ∪ B) = 2n(A) can be rewritten as:
n(A) + n(B) - n(A ∩ B) = 2n(A)
Simplifying this equation, we get:
n(B) - n(A ∩ B) = n(A)
This equation only implies that the size of B minus the size of the intersection of A and B is equal to the size of A. It does not necessarily imply that A and B have the same number of elements.
For example, consider A = {1, 2, 3} and B = {2, 3, 4}. Then, A ∪ B = {1, 2, 3, 4}, and n(A ∪ B) = 4. Also, n(A) = 3. Substituting these values in the given equation, we get:
4 = 2(3)
which is true. However, A and B are not equivalent sets as |A| = 3 and |B| = 3.
Therefore, the given equation does not necessarily imply that A and B are equivalent sets.