Geometry basic rules

Martin McBride

Here are some basic rules and definitions of geometry that are useful to know. They are also covered in this video:

Angles around a point

There are several geometry rules for angles.

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°

Angles in a quadrilateral

The interior angles in a quadrilateral (any 4-sided shape) add up to 360°.

a + b + c + d = 360°

Exterior angles of a triangle

There are a number of geometry rules for triangles.

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

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