How do I show that the function [math]f(x_1, x_2, x_3) = \begin {cases} \frac{x_1 x_2 - x_{3}^{2}}{x_{1}^{2}, x_{2}^{2}, x_{3}^{2}} & \text {if $(x_1, x_2, x_3) \neq (0,0,0)$} \\ 0 & \text {if $(x_1, x_2, x_3) = (0,0,0)$} \end {cases}?
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