Find the largest prime $p$ such that there exist positive integers $a,b$ satisfying $$p=\frac{b}{2}\sqrt{\frac{a-b}{a+b}}.$$
1. Given the equation:
   \[
   p = \frac{b}{2} \sqrt{\frac{a-b}{a+b}}
   \]
   we start by squaring both sides to eliminate the square root:
   \[
   p^2 = \left(\frac{b}{2}\right)^2 \left(\frac{a-b}{a+b}\right)
   \]
   Simplifying, we get:
   \[
   4p^2 = b^2 \frac{a-b}{a+b}
   \]
   Multiplying both sides by \(a+b\) to clear the fraction:
   \[
   4p^2(a+b) = b^2(a-b)
   \]

2. Rearrange the equation to isolate terms involving \(a\):
   \[
   4p^2(a+b) = b^2(a-b) \implies b^2(a-b) = 4p^2(a+b)
   \]
   \[
   b^2a - b^3 = 4p^2a + 4p^2b
   \]
   \[
   b^2a - 4p^2a = b^3 + 4p^2b
   \]
   \[
   a(b^2 - 4p^2) = b(b^2 + 4p^2)
   \]
   \[
   a = \frac{b(b^2 + 4p^2)}{b^2 - 4p^2}
   \]

3. Let \(a - b = l\). Then:
   \[
   a = b + l
   \]
   Substituting \(a = b + l\) into the equation:
   \[
   b(b^2 + 4p^2) = (b + l)(b^2 - 4p^2)
   \]
   \[
   b^3 + 4p^2b = b^3 - 4p^2b + lb^2 - 4p^2l
   \]
   \[
   8p^2b = lb^2 - 4p^2l
   \]
   \[
   lb^2 - 8p^2b - 4p^2l = 0
   \]

4. Consider this as a quadratic equation in \(b\):
   \[
   lb^2 - 8p^2b - 4p^2l = 0
   \]
   The discriminant of this quadratic equation must be a perfect square:
   \[
   \Delta = (8p^2)^2 + 4 \cdot l \cdot (-4p^2l) = 64p^4 + 16p^2l^2 = 16p^2(4p^2 + l^2)
   \]
   For \(b\) to be an integer, \(4p^2 + l^2\) must be a perfect square. Let:
   \[
   4p^2 + l^2 = \Delta^2
   \]
   Then:
   \[
   4p^2 = (\Delta - l)(\Delta + l)
   \]

5. Let \(\Delta - l = x\) and \(\Delta + l = y\). Then:
   \[
   y - x = 2l \quad \text{and} \quad xy = 4p^2
   \]
   Possible pairs \((x, y)\) are:
   \[
   (1, 4p^2), (2, 2p), (4, p^2), (p, 4p), (2p, 2p), (1, 4p), (4, p^2), (2, 2p^2), (1, 4p^2)
   \]

6. This implies that \(l = p - 1\) or \(l = p^2 - 1\). Checking \(l = p - 1\):
   \[
   4p^2 + (p - 1)^2 = \Delta^2
   \]
   \[
   4p^2 + p^2 - 2p + 1 = \Delta^2
   \]
   \[
   5p^2 - 2p + 1 = \Delta^2
   \]
   This does not yield a perfect square for any prime \(p\).

7. Checking \(l = p^2 - 1\):
   \[
   4p^2 + (p^2 - 1)^2 = (p^2 + 1)^2 = \Delta^2
   \]
   Therefore:
   \[
   b = \frac{8p^2 \pm 4p(p^2 + 1)}{2(p^2 - 1)} = \frac{4p^2 \pm 2p(p^2 + 1)}{p^2 - 1}
   \]
   Simplifying:
   \[
   b = \frac{4p^2 + 2p^3 + 2p}{p^2 - 1} = 2p + 4 + \frac{4}{p - 1}
   \]
   This gives us \(p = 5\) or \(p = 3\).

8. If \(b = \frac{4p^2 - 2p^3 - 2p}{p^2 - 1}\), then \(b\) is not a positive integer, which is a contradiction.

The final answer is \( \boxed{ (p; a; b) = (5; 39; 15) } \)