Let $a,b,c>0$ the sides of a right triangle. Find all real $x$ for which $a^x>b^x+c^x$, with $a$ is the longest side.
1. Given that \(a, b, c > 0\) are the sides of a right triangle, and \(a\) is the hypotenuse (the longest side). By the Pythagorean theorem, we have:
   \[
   a^2 = b^2 + c^2
   \]

2. We need to find all real \(x\) for which \(a^x > b^x + c^x\).

3. First, consider the case when \(x = 2\):
   \[
   a^2 = b^2 + c^2
   \]
   This shows that the equality holds at \(x = 2\).

4. Now, consider \(x > 2\). We need to show that \(a^x > b^x + c^x\). Using the fact that \(a^2 = b^2 + c^2\), we can write:
   \[
   a^x = (a^2)^{x/2} = (b^2 + c^2)^{x/2}
   \]

5. Since \(a\) is the longest side, we have \(a > b\) and \(a > c\). For \(x > 2\), the function \(f(t) = t^{x/2}\) is convex for \(t > 0\). By the convexity of the function, we have:
   \[
   (b^2 + c^2)^{x/2} > b^x + c^x
   \]
   This is because the sum of the \(x/2\)-th powers of \(b\) and \(c\) is less than the \(x/2\)-th power of their sum.

6. Therefore, for \(x > 2\), we have:
   \[
   a^x = (b^2 + c^2)^{x/2} > b^x + c^x
   \]

7. For \(x < 2\), consider the behavior of the function \(f(t) = t^{x/2}\). For \(x < 2\), the function \(f(t)\) is concave for \(t > 0\). By the concavity of the function, we have:
   \[
   (b^2 + c^2)^{x/2} < b^x + c^x
   \]
   This implies that for \(x < 2\), \(a^x < b^x + c^x\).

8. Therefore, the inequality \(a^x > b^x + c^x\) holds for \(x > 2\).

\(\blacksquare\)

The final answer is \( \boxed{ x > 2 } \)