[dektep]

$0 < \frac{\left(x+y+z\right)\left(xy+yz+zx\right)}{\left(x+y\right)\left(y+z\right)\left(z+x\right)} \le \frac{9}{8}$

ซึ่งจริงโดย am-gm ; $\sum_{sym}a^2b \ge 6abc$