If $\triangle ABC$ is right angled at A, where $A\equiv(4,2,x)$, $B\equiv(3,1,8)$ and $C\equiv(2,-1,2)$, then the value of $x$ is
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Geometry Tip: When dealing with angles in 3D coordinate geometry, converting the points to vectors and utilizing the dot product is almost always the fastest method.Two vectors are perpendicular if and only if their dot product is 0.