Given a positive integer $n \geq 3$. Find all $f:\mathbb{R}^+ \rightarrow \mathbb{R}^+$ such that for any $n$ positive reals $a_1,...,a_n$, the following condition is always satisfied:
$\sum_{i=1}^{n}(a_i-a_{i+1})f(a_i+a_{i+1}) = 0$
where $a_{n+1} = a_1$.
1. We start with the given condition for any \( n \) positive reals \( a_1, a_2, \ldots, a_n \):
   \[
   \sum_{i=1}^{n} (a_i - a_{i+1}) f(a_i + a_{i+1}) = 0
   \]
   where \( a_{n+1} = a_1 \).

2. Note that if \( S_n \) is the set of functions that satisfy the above condition for \( n \) positive reals, then \( S_n \subseteq S_{n-1} \). Therefore, it suffices to consider the case when \( n = 3 \).

3. For \( n = 3 \), the condition becomes:
   \[
   (a - b) f(a + b) + (b - c) f(b + c) + (c - a) f(a + c) = 0
   \]
   Let \( P(a, b, c) \) denote this assertion.

4. Consider the specific case \( P(a, a+d, a+2d) \):
   \[
   (a - (a+d)) f(a + (a+d)) + ((a+d) - (a+2d)) f((a+d) + (a+2d)) + ((a+2d) - a) f((a+2d) + a) = 0
   \]
   Simplifying, we get:
   \[
   -d f(2a + d) - d f(2a + 3d) + 2d f(2a + 2d) = 0
   \]
   Dividing by \( d \) (assuming \( d \neq 0 \)):
   \[
   -f(2a + d) - f(2a + 3d) + 2f(2a + 2d) = 0
   \]
   This simplifies to:
   \[
   f(2a + d) + f(2a + 3d) = 2f(2a + 2d)
   \]

5. The equation \( f(x + y) = 2f\left(\frac{x + y}{2}\right) \) is a form of Jensen's functional equation. The general solution to this equation is \( f(x) = kx \) for some constant \( k \).

6. To verify, substitute \( f(x) = kx \) back into the original condition:
   \[
   (a - b) k (a + b) + (b - c) k (b + c) + (c - a) k (a + c) = 0
   \]
   Simplifying, we get:
   \[
   k \left[ (a - b)(a + b) + (b - c)(b + c) + (c - a)(a + c) \right] = 0
   \]
   Expanding each term:
   \[
   k \left[ a^2 - b^2 + b^2 - c^2 + c^2 - a^2 \right] = 0
   \]
   This simplifies to:
   \[
   k \cdot 0 = 0
   \]
   which is always true.

7. Therefore, the solution \( f(x) = kx \) satisfies the given condition for any \( n \geq 3 \).

The final answer is \( \boxed{ f(x) = kx } \) for some constant \( k \).