**
Straight line
**

*
Equation of straight line going
through two different points.
*

*
Parametric equation of straight
line.
*

*
Equation of intersection line of
two planes.
*

*
Parallelism condition of
straight lines.
*

*
Perpendicularity condition of
straight lines.
*

*
Angle between straight lines.
*

*
Angle between straight line and
plane.
*

*
*

**
An equation of straight line
going through two different points
**
(

*õ*

_{ 1 },

*ó*

_{ 1 },

*z*

_{ 1 }) and (

*õ*

_{ 2 },

*ó*

_{ 2 },

*z*

_{ 2 }):

**
A parametric equation of
straight line
**
, going
through a point
(

*õ*

_{ 0 },

*ó*

_{ 0 }

**,**

*z*

_{ 0 }) and parallel to a

*direction vector*(

*a*,

*b*,

*ñ*) of a straight line:

Let two planes
**
**

*Àõ + Âó + Ñ z+ D =*0 and

*E õ + F ó + Gz+ H =*0 be given, moreover, their normal vectors aren't collinear, then the set of simultaneous equations

describes
**
an intersection line
**
of these planes.

**
**

Let (
*
a
*
,
*
b
*
,
*
ñ
*
) and (
*
p
*
,
*
q
*
,
*
r
*
) be direction vectors of two straight lines, then
**
a parallelism condition of straight
lines
**
is :

*
aq
*
–
*
bp
*
=
*
br
*
–
*
cq
*
=
*
ar
*
–
*
cp
*
= 0 ,

**
a perpendicularity condition
of straight lines
**
is
:

*
ap
*
+
*
bq
*
+
*
cr
*
= 0 ,

**
**

**
an
angle
**

*between straight lines:*

**
an
angle
**

*between a straight line and a plane:*