Hello,
A mediator is perpendicular to the segment and passes throught the midpoint.
The coordinates of are
So the equation of the plan, which has as orthogonal vector, will be :
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With d to determine. And for this, you will have to use the fact that it goes throught the midpoint of AB.
Coordinates of the midpoint of [AB] : , that is to say .
This point is on the plane, so substitute and get d.
After that, divide the equation by 2.