Let P be the point with position vector and M be the image of P in the plane.
In addition, let Q be the foot of the perpendicular from P on to the given plane. So, Q is the midpoint of PM.
Direction ratios of PM are proportional to 2, –1, 1 as PM is normal to the plane and parallel to.
Recall the vector equation of the line passing through the point with position vector and parallel to vector is given by
Hence, the equation of PM is
Let the position vector of M be. As M is a point on this line, for some scalar α, we have
Now, let us find the position vector of Q, the midpoint of PM.
Let this be.
Using the midpoint formula, we have
This point lies on the given plane, which means this point satisfies the plane equation.
We have the image
Therefore, image is (1, 2, 1)
Foot of the perpendicular
Thus, the position vector of the image is and that of the foot of perpendicular is.
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