|mathsrevision.net --> gcse||
|21st May 2013||NOTE: - Welcome to MathsRevision - we reach over 1 million users each month! - NEW SITE COMING SOON|
Shape and Space
Statistics and Probability
Angles are measured in degrees, written °. The maximum angle is 360°. This is the angle all the way round a point. Half of this is the angle on a straight line, which is 180°.
Lines AB and CD are parallel to one another (hence the » on
Angle Sum of a Triangle
Using some of the above results, we can prove that the sum of the three angles inside any triangle always add up to 180 degrees.
If we have a triangle, you can always draw two parallel lines like this:
Now, we know that alternate angles are equal. Therefore the two angles labelled x are equal. Also, the two angles labelled y are equal.
We know that x, y and z together add up to 180 degrees, because these together is just the angle around the straight line. So the three angles in the triangle must add up to 180 degrees.
Angle Sum of a Quadrilateral
A quadrilateral is a shape with 4 sides.
Now that we know the sum of the angles in a triangle, we can work out the sum of the angles in a quadrilateral.
For any quadrilateral, we can draw a diagonal line to divide it into two triangles. Each triangle has an angle sum of 180 degrees. Therefore the total angle sum of the quadrilateral is 360 degrees.
The exterior angles of a shape are the angles you get if you extend the sides. The exterior angles of a hexagon are shown:
A polygon is a shape with straight sides. All of the exterior angles of a polygon add up to 360°. because if you put them all together they form the angle all the way round a point:
Therefore if you have a regular polygon (in other words, where all the sides are the same length and all the angles are the same), each of the exterior angles will have size 360 ÷ the number of sides. So, for example, each of the exterior angles of a hexagon are 360/6 = 60°.
The interior angles of a shape are the angles inside it. If you know the size of an exterior angle, you can work out the size of the interior angle next to it, because they will add up to 180° (since together they are the angle on a straight line).
Exterior Angle of a Triangle
Angle x is an exterior angle of the triangle:
The exterior angle of a triangle is equal to the sum of the interior angles at the other two vertices. In other words, x = a + b in the diagram.