# Space

## A plane is determined by

1. 3 non-collinear points
2. A line and a point outside the plane
3. 2 intersecting lines
4. 2 parallel lines

## How to prove a line perpendicular to a plane ?

If a straight line is perpendicular to 2 intersecting lines in a plane then its perpendicular to the plane itself.
When Is a Line Perpendicular to a Plane? : Trigonometry & Other Math

## Relative position of a plane and a line :

1. Intersecting
2. Parallel
3. The line is in the plane

## Relative position of 2 lines :

1. Skew
2. Parallel
3. Intersecting
4. Confounded

## Relative position of 2 planes :

1. Intersecting
2. Parallel
3. Confounded

## The Steps :

1- Find the common line.

2- Select 2 lines perpendicular from each plane to the common line.

3- Find the angle.

## Mediator Plane

Property : Any point on the mediator plane of a line is equidistant from the extremities of this line.

## How to prove a mediator plane to a line ?

Prove that it has 3 points equidistant from the extremities of the line.

## The Angular bisector

Property : Any point on the angular bisector of an angle is equidistant from the sides of the lines forming this angle.

## How to prove 3 collinear points?

Prove that they belong to a line that is the intersection of 2 planes