If $x, y, z$ are three distinct positive integers, where $x + y + z = 13$ and $xy, xz, yz$ form an arithmetic... If $x, y, z$ are three distinct positive integers, where $x + y + z = 13$ and $xy, xz, yz$ form an increasing arithmetic sequence, what is the value of $(x + y)^z$ ?
I've been trying to solve this by forming equations based off of formulas ($yz - xz = xz - xy$) and by trying to clean it up by systems of equations (I did $(13 - z)^2 = 13$), but I can't seem to find the solution. 

This is actually not too bad to brute force since $x + y + z = 13$ and they are all positive integers. There are actually only $11$ possible values for $x$: $1, 2, 3, \ldots, 11$.
However, with a bit of analysis, we can reduce the number of cases.
Since $xz - xy = yz - xz$, we have that $2xz = xy + yz = y(x+z) = y(13-y)$. Since
\begin{align*}
(x+z)^2 \ge 4xz = 2y(13-y),
\end{align*}
we have that
\begin{align*}
(13-y)^2 \ge 2y(13-y).
\end{align*}
Since $0 < y < 13$ (otherwise $x = z = 0$ which is impossible since $x$ and $z$ are positive integers), then $13 - y > 0$ so we can divide both sides by $13-y$ without changing the sign to get
\begin{align*}
13-y \ge 2y,
\end{align*}
so this implies $y \le 4$. We have 4 cases to consider.


*

*If $y = 1$, then $x + z = 12$ and $xz = 6$. No solution.

*If $y = 2$, then $x + z = 11$ and $xz = 11$. No solution.

*If $y = 3$, then $x + z = 10$ and $xz = 15$. No solution.

*If $y = 4$, then $x + z = 9$ and $xz = 18$. This gives $(x,z) = (6,3)$ or $(3,6)$.


Hence, $(x+y)^z$ is either $(6+4)^3 = 1000$ or $(3+4)^6 = 117649$.
