The physical quantities which have magnitude, as well as directly attached to them, are known as vectors. Position vectors simply denote the position or location of a point in the three dimensional Cartesian systems with respect to a reference origin. In the upcoming discussion, we shall study about vectors and the Cartesian equation of a plane in three dimensions.

In the previous section we have already discussed that to a given vector there lie infinite planes which are perpendicular to it. But for a particular point there exist one and only one unique plane which is perpendicular to it passing through the given point. The vector equation of a plane in this case is given as

\((\vec{r} – \vec{a}) . \vec{N} = 0\)

Here, \(\vec{r} \)

The Cartesian equation of such a plane is represented by:

\(A(x-{x}_{1}) + B (y- {y}_{1}) + C (z -{z}_{1}) = 0 \)

Here, A, B, and C are the direction ratios.

Let us now discuss the equation of a plane passing through three points which are non-collinear.

### Equation of a plane passing through three non collinear points

Let us consider three non collinear points P, Q, R lying on a plane such that their position vectors are given by \(\vec{a} \)

The vectors \(\vec{PQ} \)

(\(\vec{r} \)

Also, from the above figure, and . Substituting these values in the above equation, we have

\((\vec{r} – \vec{a}).[(\vec{b}- \vec{a}) × (\vec{c}- \vec{a})] = 0 \)

This represents the equation of a plane in vector form passing through three points which are non- collinear.

To convert this equation in Cartesian system, let us assume that the coordinates of the point P, Q and R are given as (x_{1}, y_{1}, z_{1}), (x_{2}, y_{2}, z_{2}) and (x_{3}, y_{3}, z_{3}) respectively. Also let the coordinates of point A be x, y and z.

Substituting these values in the equation of a plane in Cartesian form passing through three non-collinear points, we have

This is the equation of a plane in Cartesian form passing through three points which are non- collinear.

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