# Around your space

## Orthogonality

- (d) _l_ (d') when their angle = 90
- if (d) _l_ (d') any line // to one is _l_ to the other
**a st. line (d) is _l_ to plane (p) if and only if it is _l_ to 2 int st lines contained in (p)**- 2 planes are _l_ if their angle= 90
- through a given pt one and only one st line can be drawn _l_ to a given plane
**if (d) _l_ to plane (p) then (d)_l_(a) any line in (p)**

## How to get an angle ?

The acute angle of a plane (p) and a st line (R) is the angle HIA where h is the the orthogonal proj of any pt A of (R) on (p)__angle of a st line and a plane :__

## What is the mediator plane ?

**Mediator plane**of

**[AB]**plane

**_l_**to

**[AB]**at its

**midpt**

- Any pt of mediator plane is equidistant from extremeties