For a sequence $a_1,a_2,...,a_m$ of real numbers, define the following sets 
\[A=\{a_i | 1\leq i\leq m\}\  \text{and} \ B=\{a_i+2a_j | 1\leq i,j\leq m, i\neq j\}\]
Let $n$ be a given integer, and $n>2$. For any strictly increasing arithmetic sequence of positive integers, determine, with proof, the minimum number of elements of set $A\triangle B$, where $A\triangle B$ $= \left(A\cup B\right) \setminus \left(A\cap B\right).$
To determine the minimum number of elements of the set \( A \triangle B \) for a strictly increasing arithmetic sequence of positive integers, we need to analyze the sets \( A \) and \( B \) and their symmetric difference.

1. **Define the sequence and sets:**
   Let \( a_1, a_2, \ldots, a_m \) be a strictly increasing arithmetic sequence of positive integers. This means \( a_i = a_1 + (i-1)d \) for some common difference \( d > 0 \).

   The set \( A \) is defined as:
   \[
   A = \{a_i \mid 1 \leq i \leq m\}
   \]
   The set \( B \) is defined as:
   \[
   B = \{a_i + 2a_j \mid 1 \leq i, j \leq m, i \neq j\}
   \]

2. **Calculate the elements of \( B \):**
   For \( i \neq j \), the elements of \( B \) are of the form:
   \[
   a_i + 2a_j = (a_1 + (i-1)d) + 2(a_1 + (j-1)d) = 3a_1 + (2j + i - 3)d
   \]
   Since \( i \neq j \), the values \( 2j + i - 3 \) will vary, leading to different elements in \( B \).

3. **Determine the size of \( B \):**
   The number of distinct elements in \( B \) depends on the number of distinct values \( 2j + i - 3 \) can take. For \( m \geq 3 \), there are \( m(m-1) \) possible pairs \((i, j)\) with \( i \neq j \).

4. **Analyze \( A \cap B \):**
   The intersection \( A \cap B \) consists of elements that are both in \( A \) and \( B \). Since \( A \) is a strictly increasing arithmetic sequence, and \( B \) consists of sums of the form \( a_i + 2a_j \), it is unlikely that many elements of \( A \) will be in \( B \).

5. **Calculate \( |A \triangle B| \):**
   The symmetric difference \( A \triangle B \) is given by:
   \[
   |A \triangle B| = |A| + |B| - 2|A \cap B|
   \]
   For \( m = 2 \):
   - \( A = \{a_1, a_2\} \)
   - \( B = \{a_1 + 2a_2, a_2 + 2a_1\} \)
   - Since \( a_1 \) and \( a_2 \) are distinct, \( A \cap B = \emptyset \)
   - Thus, \( |A \triangle B| = 2 + 2 - 0 = 4 \)

   For \( m \geq 3 \):
   - \( |A| = m \)
   - \( |B| \geq 3m - 2 \) (as shown in the solution)
   - \( |A \cap B| \) is minimal, so we approximate it as 0 for the lower bound
   - Thus, \( |A \triangle B| \geq m + (3m - 2) - 2 \cdot 0 = 4m - 2 \)

6. **Conclusion:**
   The minimum number of elements of \( A \triangle B \) is:
   \[
   \boxed{2m - 2 \text{ for } m \geq 3}
   \]
   For \( m = 2 \), the minimum number is \( 0 \).