Show that the vectors `vec a - vec b, vec b - vec c , vec c - vec a` are coplaner.

Explanation

$$ \begin{array}{ll} \vec{A}=(\vec{a}-\vec{b}) & =(\hat{\imath}-\hat{1}) \\ \overrightarrow{\vec{b}}=(\vec{b}-\vec{c}) & =(\hat{\imath}-\hat{k}) \\ \vec{C}=(\vec{c}-\vec{a}) & =(\hat{k}-\hat{\imath}) \end{array} $$
$$[\vec{A} \cdot(\vec{B} \times \vec{C})]=0$$
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