Find all integer solutions to the equation $\frac{xy}{z}+\frac{xz}{y}+\frac{zy}{x}=15$ I was going through one of my Mathematics books and I came to this problem:
Find all integer solutions to the equation: $\frac{xy}{z}+\frac{xz}{y}+\frac{zy}{x}=15$
I tried a few things to begin with but none went in the right direction, any suggestions? 
Another thing, if we have the left side of the equation to be equal to 9, there are supposed to be 16 solutions in integers. What way can they be found? 

*

*First, let us suppose that $x,y,z$ are positive integers satisfying the proposed equality. Then
$$ xyz(15-x-y-z)=\frac{1}{2}\left((xy-yz)^2+(yz-zx)^2+(zx-xy)^2\right)\ge0$$
So we have $x+y+z\le 15$.

*Now, note that squares modulo 5 are $\{0,-1,1\}$. Thus, if the sum of three squares modulo $5 $ is $0$ then either all three of them are multiples of $5$, (the case $0+0+0$) or just exactly one of them is a multiple of $5$ and the other two are equal to $+1$ and $-1$ modulo $5$, ( the case $0+1-1$). In both cases $5$ divides at least one of the three numbers.
The equality
$$(xy)^2+(yz)^2+(zx)^2=15xyz\equiv 0\mod 5\tag1$$
shows then that one of the numbers $xy,yz,zx$ is a multiple of $5$. So, we may and will suppose that $x=5a$.  But then $(1)$ proves that $5$ must divide $yz$ and again we may suppose that $ y=5 b$.

*Now, $5+5+1\le 5a+5b+z\le15$ implies that $ a=b=1$, that is $x=y=5$ and the equation implies then that $z=5$.
We conclude that $(5,5,5)$ is the only solution consisting of positive integers.

*The general case. Suppose that $x=sx'$,$y=ty'$ and $z=uz'$ is a solution to the proposed equation where
$x',y',z'$ are positive integers, and $s,t,u$ are $\pm1$. It follows from
$$xyz\left(\frac{1}{x^2}+ \frac{1}{y^2}+\frac{1}{z^2}\right)=15$$
That
$$stu x'y'z'\left(\frac{1}{x'^2}+ \frac{1}{y'^2}+\frac{1}{z'^2}\right)=15$$
This implies that $stu=1$ and
$$ x'y'z'\left(\frac{1}{x'^2}+ \frac{1}{y'^2}+\frac{1}{z'^2}\right)=15$$
So, $x'=y'=z'=5$ and exactly two of $s,t,u$ are equal $-1$, or they are all equal to $+1$. This gives the following set of solutions $$\{(5,5,5),(-5,-5,5),(-5,5,-5),(5,-5,-5)\}.$$

