Find all triples $(a, b, n)$ of positive integers such that $a$ and $b$ are both divisors of $n$, and $a+b = \frac{n}{2}$ .
1. **Case 1: \( a = b \)**

   Given that \( a = b \), we substitute into the equation \( a + b = \frac{n}{2} \):
   \[
   a + a = \frac{n}{2} \implies 2a = \frac{n}{2} \implies n = 4a
   \]
   Therefore, the triple \((a, b, n)\) in this case is:
   \[
   (a, a, 4a)
   \]
   Let \( t = a \). Then the solution can be written as:
   \[
   (t, t, 4t)
   \]
   where \( t \in \mathbb{N}_+ \).

2. **Case 2: \( a \neq b \)**

   Without loss of generality, assume \( a > b \). Given \( a + b = \frac{n}{2} \), we need to find the relationship between \( a \) and \( b \) such that both are divisors of \( n \).

   Since \( a \) and \( b \) are divisors of \( n \), \( n = ka \) and \( n = lb \) for some integers \( k \) and \( l \). From \( a + b = \frac{n}{2} \), we have:
   \[
   a + b = \frac{n}{2} \implies 2(a + b) = n
   \]
   Since \( a \mid 2(a + b) \), it follows that \( a \mid 2b \). Given \( a > b \), the only possibility is \( a = 2b \).

   Substituting \( a = 2b \) into the equation \( a + b = \frac{n}{2} \):
   \[
   2b + b = \frac{n}{2} \implies 3b = \frac{n}{2} \implies n = 6b
   \]
   Therefore, the triple \((a, b, n)\) in this case is:
   \[
   (2b, b, 6b)
   \]
   Let \( t = b \). Then the solution can be written as:
   \[
   (2t, t, 6t)
   \]
   where \( t \in \mathbb{N}_+ \).

   Similarly, if we consider \( b > a \), we would get the same form of solution by symmetry:
   \[
   (t, 2t, 6t)
   \]
   where \( t \in \mathbb{N}_+ \).

In conclusion, the solutions are:
\[
(a, b, n) = (t, t, 4t), (2t, t, 6t), (t, 2t, 6t) \quad \text{for} \quad t \in \mathbb{N}_+
\]

The final answer is \( \boxed{ (t, t, 4t), (2t, t, 6t), (t, 2t, 6t) } \) for \( t \in \mathbb{N}_+ \).