Three Dimensional Geometry Class XII Chapter 11
Basic concepts and formulas of 3DGeometry class XII chapter 11, Equations of line and plane in space, shortest distance between skew lines, angle between two lines and planes
Introduction:
It is that branch of mathematics in which we discuss the point, line and plane in the space.
We got a little introduction of 3 dimensional Geometry in class 11 chapter 12.
Here in class XII we make use of vector algebra in 3 dimensional  Geometry.
In this chapter we discuss the following points.

 Direction angles,
 Direction cosines
 Direction ratios.
 Equation of line passing through one point, two point.
 Angle between two lines, parallel lines and perpendicular lines.
 Shortest distance between two lines.
 Collinearity of two lines.
 Equation of plane passing through one point and through three points.
 Angle between two planes.
 Distance of the plane from the point.
 Angle between line and plane.
Direction angles:
These are the angles made by the vector with the positive direction of the axis. These are denoted by α, β, 𝜸
Direction cosines:
Cosines of the direction angles are called direction cosines.
If α, β, 𝜸 are the direction angles made by a vector with the axis then cosα, cosβ, cos𝜸 are called the direction cosines. These are also denoted by l, m, n
l = cosα, m = cosβ, n = cos𝜸
If l, m, n are the direction cosines of a line then l^{2 }+ m^{2 }+ n^{2}
= 1
Direction ratios:
The terms which are proportional to the direction cosines are called direction ratios. These are denoted by (a, b, c)
For Example:\[let\:
any\: \: vector,\: \: \overrightarrow{r}=a\hat{i}+b\hat{j}+c\hat{k}\]\[Its\;
magnitude\; is\; \; \overrightarrow{r}=\sqrt{a^{2}+b^{2}+c^{2}}\](a, b, c)
are the direction ratios and the direction cosines are given by
\[\left (
\frac{a}{\sqrt{a^{2}+b^{2}+c^{2}}},\frac{b}{\sqrt{a^{2}+b^{2}+c^{2}}},\frac{c}{\sqrt{a^{2}+b^{2}+c^{2}}}
\right )\]

If ɑ, 𝛃, 𝛄 are the direction angles made
by the line with positive direction of xaxis, yaxis, zaxis then cosɑ ,
cos𝛃 , and cos𝛄 are called the direction
cosines of that line. These are also be denoted by l, m, n such
that
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EQUATION OF LINES IN SPACE

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Case I : Equation of a line passing through the point A(x_{1}, y_{1}, z_{1}) and is parallel to vector b is given by
In vector form
\[\; \; \vec{r}=\vec{a}+\lambda \vec{b}\]

\[Where\; \; \; \vec{r}=x\hat{i}+y\hat{j}+z\hat{k}\]
\[\vec{a}=x_{1}\hat{i}+y_{1}\hat{j}+z_{1}\hat{k}\: and\: \vec{b}= a\hat{i} +b\hat{j} +c\hat{k}\]\[\vec{r}=x_{1}\hat{i}+y_{1}\hat{j}+ z_{1}\hat{k}+ \lambda(a\hat{i} +b\hat{j}+c\hat{k})\]In cartesian form
\[\frac{xx_{1}}{a}=\frac{yy_{1}}{b}=\frac{zz_{1}}{c}=\lambda\]

Case II : Equation of a line passing through the two points.
Let given equation passes through the two points A(x_{1}, y_{1}, z_{1}) and B(x_{2}, y_{2}, z_{2}). Let P(x, y, z) is any arbitrary point on the line.
Then equation of line is given by
In vector form
\[\; \;
\vec{r}=\vec{a}+\lambda (\vec{b}\vec{a})\]

\[Where\; \; \; \vec{r}=x\hat{i}+y\hat{j}+z\hat{k}\]
\[\vec{a}=x_{1}\hat{i}+y_{1}\hat{j}+z_{1}\hat{k}\]
\[\vec{b}=x_{2}\hat{i}+y_{2}\hat{j}+z_{2}\hat{k}\]
\[\vec{r}=x_{1}\hat{i}+y_{1}\hat{j}+z_{1}\hat{k}+\lambda \left [ (x_{2}x_{1})\hat{i}+(y_{2}y_{1})\hat{j}+(z_{2}z_{1})\hat{k} \right ]\]In cartesian form
\[\frac{xx_{1}}{x_{2}x_{1}}=\frac{yy_{1}}{y_{2}y_{1}}=\frac{zz_{1}}{z_{2}z_{1}}=\lambda\]

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In Vector form let two equations of lines are
\[\vec{r}=\vec{a_{1}}+\lambda \vec{b_{1}}\; and\; \; \vec{r}=\vec{a_{2}}+\lambda \vec{b_{2}}\]and let θ is the angle between these two lines then we have
\[cos\theta =\left \frac{\overrightarrow{b_{1}}.\overrightarrow{b_{2}}}{\overrightarrow{b_{1}}\overrightarrow{b_{2}}}
\right \]

In cartesian form let two equations of line are\[\frac{xx_{1}}{a_{1}}=\frac{yy_{1}}{b_{2}}=\frac{zz_{1}}{c_{1}}=\lambda\:\: and\\\frac{xx_{1}}{a_{2}}\: =\: \frac{yy_{1}}{b_{2}}\: =\: \frac{zz_{1}}{c_{2}}=\mu\:\:then\]
\[cos\theta =\left \frac{a_{1}a_{2}+ b_{1}b_{2}
+c_{1}c_{2}}{\sqrt{(a_{1}^{2}+b_{1}^{2}+c_{1}^{2})(a_{2}^{2}+ b_{2}^{2}+
c_{2}^{2})}} \right \]\[Two\: lines\: are\: perpendicular\: \: if\: \:
{a_{1}a_{2} +b_{1}b_{2}+c_{1}c_{2}}=0\]\[Two\: lines\: are\: Parallel\: \:
if\: \: \frac{a_{1}}{a_{2}}=\frac{b_{1}}{b_{2}}=\frac{c_{1}}{c_{2}}\]

Skew lines:
In space there are lines which are neither parallel nor intersecting such lines are noncoplanar and are called skew lines.
Shortest Distance
between two lines

If two lines in space intersect each other then shortest distance between them = 0,
If two lines in one plane are parallel then shortest distance is the perpendicular distance between them.
Let two skew lines in
vector form are \[\overrightarrow{r}=\overrightarrow{a_{1}}+\lambda
\overrightarrow{b_{1}}\; and\;
\overrightarrow{r}=\overrightarrow{a_{2}}+\lambda
\overrightarrow{b_{2}}\]Shortest distance between two lines is
\[d=\left  \frac{\left
(\overrightarrow{a_{2}}\overrightarrow{a_{1}} \right ).\left ( \overrightarrow{b_{1}}\times
\overrightarrow{b_{2}} \right )}{\left \overrightarrow{b_{1}}\times
\overrightarrow{b_{2}} \right } \right \]

Coplanarity of
two lines: Two lines are coplanar if
\[\left (\overrightarrow{a_{2}}\overrightarrow{a_{1}} \right
).\left ( \overrightarrow{b_{1}}\times \overrightarrow{b_{2}} \right )=0\]

In cartesian form two lines are coplanar if
\[ \left  \begin{matrix} x_{2}x_{1} &y_{2}y_{1}
&z_{2}z_{1} \\ a_{1}&b_{1} &c_{1} \\ a_{2} &b_{2} &c_{2}
\end{matrix} \right =0\]

Here shortest distance
between two lines become = 0 and the lines are coplanar lines or intersecting
lines.
Distance between two parallel lines

Two lines are parallel when their direction vectors are same.
When two lines are parallel then
they are coplanar lines.
Let two parallel lines are \[\overrightarrow{r}=\overrightarrow{a_{1}}+\lambda
\overrightarrow{b_{1}}\; and\;\overrightarrow{r}=\overrightarrow{a_{2}}+\lambda
\overrightarrow{b_{2}}\]\[When\; two\; lines\; are\; parallel\; then\; \; \overrightarrow{b_{1}} =\overrightarrow{b_{2}}=\overrightarrow{b}\]
Then shortest distance between two parallel lines is
\[d=\left \frac{
(\overrightarrow{a_{2}}\overrightarrow{a_{1}})\times
\overrightarrow{b}}{\left  b \right } \right \]

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THREE DIMENTINAL GEOMETRY
PLANE

A plane is determined uniquely if any one of the following is known.
a) When normal to the plane and its distance from the origin is given. That is equation of the plane in Normal form.
b) When it passes through a point and is perpendicular to the given direction.
c) It passes through the three non collinear points.
Normal form of equation of Plane

Let us consider a plane whose ⟂ distance from the origin is d. Let P(x, y, z) is any arbitrary point in the plane whose position vector from the origin is vector r.
Equation of plane in normal form is
given as
\[In\: vector\: form\: \:\; \vec{r}.\hat{n}=d,\: \:\]

Where: \[\vec{r}=x\hat{i}+y\hat{j}+z\hat{k}\]\[\hat{n}=l\hat{i}+m\hat{j}+n\hat{k}\]here
l, m, n are the direction cosines of the normal vector N.
Equation of plane in cartesian form
is \[(x\hat{i}+y\hat{j}+z\hat{k}).(l\hat{i}+m\hat{j}+n\hat{k})=d\]
[lx+my+nz=d\]
Equation of the plane perpendicular to the given vector and passing through a given point.
There are many planes which are perpendicular to the given vector as shown in the figure.
but there is only one plane which is perpendicular to the given vector and is passes through the given point.
Let a plane passes through the point A with position vector a and is ⊥ to a vector N.
Let P(x,y,z) is an arbitrary point in the plane
In triangle AOP by triangle law of vector addition we have
\[\overrightarrow{OA}=\overrightarrow{OP}+\overrightarrow{AP}\]\[\overrightarrow{AP}=\overrightarrow{OP}\overrightarrow{OA}\]\[\overrightarrow{AP}=\overrightarrow{r}\overrightarrow{a}\]Vector N is a normal vector to the plane and is always perpendicular to the plane and vector AP is a vector in the plane so vector AP is always ⊥ to the normal vector of the plane. When two vectors are ⊥ then their dot product is zero.
\[(\vec{r}\vec{a}).\vec{N}=0,\; \Rightarrow \; \vec{r}.\vec{N}\vec{a}.\vec{N}=0\Rightarrow
\vec{r}.\vec{N}=d\]

\[Where\; \; \vec{a}=x_{1}\hat{i}+y_{1}\hat{j}+z_{1}\hat{k}\; \; is\; the\; position\; vector\; of\; a\; point\; in\; the\; plane\]\[\vec{N}=a\hat{i}+b\hat{j}+c\hat{k}\: is\: a\: vector\: perpendicular\; to\; the\; plane\]
Equation of the plane passing through the three non
collinear points.

Let vector a, vector b and vector c are three noncollinear vectors in the plane and let P(x, y, z) is any arbitrary point in the plane. Then equation of plane passing through these three points is given by
\[\overrightarrow{AB}\times \overrightarrow{AC}\;\; is \; a \; vector\; \perp\; to\; the\; plan\; and\; \; \overrightarrow{AP} \;\; lie \; in\; the\; plane\]\[So\; equation\; of\; the\; plane\; is\; given\; by\; \; \overrightarrow{AP}.[\overrightarrow{AB}\times \overrightarrow{AC}]\]\[Where \; \; \overrightarrow{AP}=\vec{r}\vec{a},\; \; \overrightarrow{AB}=\vec{b}\vec{a},\; \; \overrightarrow{AC}=\vec{c}\vec{a}\] Putting these values, the equation of the plane becomes
\[(\vec{r}\vec{a}).[(\vec{b}\vec{a})\times (\vec{c}\vec{a})]=0,\]

Where: \[\vec{r}=x\hat{i}+y\hat{j}+z\hat{k},\: \: \vec{a}=x_{1}\hat{i}+y_{1}\hat{j}+z_{1}\hat{k}\\\vec{b}=x_{2}\hat{i}+y_{2}\hat{j}+z_{2}\hat{k},\: \: \vec{a}=x_{3}\hat{i}+y_{3}\hat{j}+z_{3}\hat{k}\] In cartesian form equation of the plane passing through three points (x_{1}, y_{1}, z_{1}),
(x_{2}, y_{2}, z_{2}), (x_{3}, y_{3}, z_{3}) is given by
\[\left  \begin{matrix} xx_{1} &yy_{1} &zz_{1} \\
x_{2}x_{1}&y_{2}y_{1} &z_{2}z_{1} \\ x_{3}x_{1}& y_{3}y_{1}
&z_{3}z_{1} \end{matrix} \right =0\]

If
we apply the above formula and we get 0 = 0 ⇒ three
points are collinear.
If
three points are collinear then there are many planes passing through three
points.
Intercept form of equation of line:
\[\frac{x}{a}+\frac{y}{b}+\frac{z}{c}=1\]

Where a, b, c are the intercepts made by the plane on the xaxis, yaxis, zaxis respectively.
Equation of the plane passing through the intersection
of two planes.

In Vector form: Let equation of two panes are\[\vec{r}.\hat{n}_{1} = d_{1}\: \: and\: \:\vec{r}.\hat{n}_{2} = d_{2}\]Equation of any plane passing through the intersection of these two planes is given by
\[\vec{r}.(\hat{n}_{1}+\lambda \hat{n}_{2}) = d_{1}+\lambda
d_{2}\]

If equation of two planes are \[\vec{r}.\overrightarrow{N_{1}} = d_{1}\: \: and\: \:\vec{r}.\overrightarrow{N_{2}}=d_{2}\]Equation of any plane passing through the intersection of these two planes is given by
\[\vec{r}.(\vec{N}_{1}+\lambda \vec{N}_{2})=d_{1}+\lambda
d_{2}\]

In cartesian form let equation of two planes are \[a_{1}x+b_{1}y+c_{1}zd_{1}=0\: \: and\: \: a_{2}x+b_{2}y+c_{2}zd_{2}=0\]Equation of any plane passing through the intersection of these two planes is given by
\[(a_{1}x+b_{1}y+c_{1}zd_{1}) + \lambda (
a_{2}x+b_{2}y+c_{2}zd_{2})=0\]

Angle between two planes
Let equations of two planes are\[\vec{r}.\vec{N}_{1}=d_{1}\: \: and \: \: \: \vec{r}.\vec{N}_{2}=d_{2}\]If θ is the angle between then
\[cos\theta =\left
\frac{\overrightarrow{N_{1}}.\overrightarrow{N_{2}}}{\overrightarrow{N_{1}}\overrightarrow{N_{2}}}
\right \\\]

In cartesian form angle between two planes is given by
\[cos\theta =\left \frac{a_{1}a_{2}+ b_{1}b_{2}
+c_{1}c_{2}}{\sqrt{(a_{1}^{2}+b_{1}^{2}+c_{1}^{2})(a_{2}^{2}+ b_{2}^{2}+
c_{2}^{2})}} \right \]\[Two\: lines\: are\: perpendicular\: \: if\: \:
{a_{1}a_{2} +b_{1}b_{2}+c_{1}c_{2}}=0\]\[Two\: lines\: are\: Parallel\: \: if\: \:
\frac{a_{1}}{a_{2}}=\frac{b_{1}}{b_{2}}=\frac{c_{1}}{c_{2}}\]

Distance of a point from the plane
Distance of a point A(x_{1}, y_{1}, z_{1}) from the plane ax + by + cz = d is
\[D=\left 
\frac{ax_{1}+by_{1}+cz_{1}d}{\sqrt{a^{2}+b^{2}+c^{2}}} \right \]

In vector form
\[D=\left  \frac{\vec{a}.\vec{N}d}{\vec{a}} \right ,\: \:
where\: \vec{a}=x\hat{i}+y\hat{j}+z\hat{k}\]

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Angle between a line and a plane

\[Let \:
\: equation \: \: of\: \: line\: \: is\: \: \vec{r}=\vec{a}+\lambda \vec{b}\:
\: and\\Equation\: \: of\: plane\: \: is\: \: \vec{r}.\vec{N}=d\]Then angle
between the line and plane is the complement of the angle between the line and
normal to the plane . If θ is the angle between line and normal to the plane
then \[cos\theta =\left  \frac{\vec{b}.\vec{N}}{\vec{b}\vec{N}}
\right \]If θ is the angle between line and normal to the line
then 90θ is the angle between line and the plane.
\[cos(90\theta )=sin\theta =\left 
\frac{\vec{b}.\vec{N}}{\vec{b}\vec{N}} \right \]

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