Given: \( a \) and \( b \) are positive integers and \( a>b \). The median of a set consisting of 5 terms: \( a^2, ab, b^2, (a - b)^2 \) and \( (a + b)^2 \) is \( ab \).
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When dealing with ordered sets of terms, always ensure to compare each term's magnitude to find the correct median.
The set consists of five terms: \( a^2, ab, b^2, (a - b)^2, (a + b)^2 \). Arranging the terms in increasing order of magnitude, we find that \( ab \) is always the middle term. Therefore, the median is always \( ab \), regardless of the specific values of \( a \) and \( b \). Hence, the correct answer is "Always".