Crazy 3D processing question...

Does anyone know what the 'Approach' is ( i will figure it out from
there ) for determining if a polygon is being seen from the front or
back...???
This is useful when hiding polygons in a 3D rendering.
The input would be ( for a triangle ) ( a pentagon+ would use the
first three vertices )
[ x y z ] [ x y z ] [ x y z ]
arranged clockwise, looking into the polyhedron
and the eyeball point, ( where you're looking from, towards the
infinite horizon intersection point...??? )
5: [ x y z ]
4: [ x y z ]
3: [ x y z ]
2: [ x y z ] eyeball
1: [ x y z ] Where you're looking towards
.... eh...???
thanx.
( i used to know this, but i lost it... ! )

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