$$\sum k^{2}=\frac{1}{\Sigma x^{2}}$$
$k=\frac{1}{xΣ}$
$k=-\frac{1}{xΣ}\text{, }Σ\neq 0\text{ and }x\neq 0$
$x=\frac{1}{kΣ}$
$x=-\frac{1}{kΣ}\text{, }k\neq 0\text{ and }Σ\neq 0$
