# Geometry basic rules

Martin McBride, 2020-08-15
Tags angles in triangle angles in quadrilateral exterior angle angles on point angles around line isosceles triangle
Categories maths gcse geometry

Here are some basic geometric rules that are useful to know. They are also covered in this video:

## Angles around a point The angles around a point add up to 360°.

a + b + c + d = 360°


## Angles on a line The angles on a straight line add up to 180°.

a + b + c = 180°


## Angles in a triangle The interior angles in a triangle add up to 180°.

a + b + c = 180° The interior angles in a quadrilateral (any 4-sided shape) add up to 360°.

a + b + c + d = 360°


## Exterior angles of a triangle An exterior angle of a triangle is equal to the sum of the 2 opposite interior angles.

e = a + b


## Angles and sides of an isosceles triangle An isosceles triangle has two sides of equal length (the two sides indicated by the tick marks that are drawn across the lines). It also has two equakl angles:

a = b