Geometry basic rules
Categories: gcse geometry

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

Join the GraphicMaths Newletter
Sign up using this form to receive an email when new content is added:
Popular tags
angle area cartesian equation chord circle combinations complex polygon cosh cosine cosine rule cube decagon derivative diagonal directrix dodecagon ellipse equilateral triangle exponent exponential exterior angle focus gradient hendecagon heptagon hexagon horizontal hyperbola hyperbolic function interior angle inverse hyperbolic function irregular polygon isosceles trapezium isosceles triangle kite locus major axis minor axis newton raphson method nonagon normal octagon parabola parallelogram parametric equation pentagon perimeter permutations power pythagoras proof quadrilateral radius rectangle regular polygon rhombus root sine sine rule sinh sloping lines solving equations solving triangles square standard curves star polygon straight line graphs symmetry tangent tanh transformations trapezium triangle vertical