Two triangles are called similar if it is possible to proportionally shrink or stretch one of them to a triangle congruent to the other. Congruent triangles are similar triangles, but similar triangles are only congruent if they are the same size to begin with.
Description
Diagram
If all three pairs of corresponding angles of two triangles are equal, then the triangles are similar.
If all pairs of corresponding sides of two triangles are in proportion, then the triangles are similar.
$\frac{x}{p}=\frac{y}{q}=\frac{z}{r}$
The theorem of pythagoras
If
$\u25b5$ ABC is right-angled (
$\widehat{B}={90}^{\circ}$ ) then
${b}^{2}={a}^{2}+{c}^{2}$ Converse: If
${b}^{2}={a}^{2}+{c}^{2}$ , then
$\u25b5$ ABC is right-angled (
$\widehat{B}={90}^{\circ}$ ).
In the following figure, determine if the two triangles are congruent, then use the result to help you find the unknown letters.
$D\stackrel{\u02c6}{E}C=B\stackrel{\u02c6}{A}C={55}^{\xb0}$ (angles in a triangle add up to
${180}^{\xb0}$ ).
Calculate the unknown variables in each of the following figures. All
lengths are in mm.
State whether or not the following pairs of triangles are congruent or not.
Give reasons for your answers. If there is not enough information to make adescision, say why.
Quadrilaterals
A quadrilateral is a four sided figure. There are some special quadrilaterals (trapezium, parallelogram, kite, rhombus, square, rectangle) which you will learn about in
Geometry .
Other polygons
There are many other polygons, some of which are given in the table below.
Table of some polygons and their number of sides.
Sides
Name
5
pentagon
6
hexagon
7
heptagon
8
octagon
10
decagon
15
pentadecagon
Angles of regular polygons
Polygons need not have all sides the same. When they do, they are called regular polygons. You can calculate the size of the interior angle of a regular polygon by using:
$$\widehat{A}=\frac{n-2}{n}\times {180}^{\circ}$$
where
$n$ is the number of sides and
$\widehat{A}$ is any angle.
Find the size of the interior angles of a regular octagon.
Make sure you know what the following terms mean: quadrilaterals, vertices, sides, angles, parallel lines, perpendicular lines,diagonals, bisectors and transversals.
The properties of triangles has been covered.
Congruency and similarity of triangles
Angles can be classified as acute, right, obtuse, straight, reflex or revolution
Theorem of Pythagoras which is used to calculate the lengths of sides of a right-angled triangle
Angles:
Acute angle: An angle
${0}^{\circ}$ and
${90}^{\circ}$
Right angle: An angle measuring
${90}^{\circ}$
Obtuse angle: An angle
${90}^{\circ}$ and
${180}^{\circ}$
Straight angle: An angle measuring
${180}^{\circ}$
Reflex angle: An angle
${180}^{\circ}$ and
${360}^{\circ}$
Revolution: An angle measuring
${360}^{\circ}$
There are several properties of angles and some special names for these
There are four types of triangles: Equilateral, isoceles, right-angled and scalene
The angles in a triangle add up to
${180}^{\circ}$
Exercises
Find all the pairs of parallel lines in the following figures, giving reasons in each case.
Find angles
$a$ ,
$b$ ,
$c$ and
$d$ in each case, giving reasons.
Say which of the following pairs of triangles are congruent with reasons.
Identify the types of angles shown below (e.g. acute/obtuse etc):
Calculate the size of the third angle (x) in each of the diagrams below:
Name each of the shapes/polygons, state how many sides each has and whether it is regular (equiangular and equilateral) or not:
Assess whether the following statements are true or false. If the statement is false, explain why:
An angle is formed when two straight lines meet at a point.
The smallest angle that can be drawn is 5°.
An angle of 90° is called a square angle.
Two angles whose sum is 180° are called supplementary angles.
Two parallel lines will never intersect.
A regular polygon has equal angles but not equal sides.
An isoceles triangle has three equal sides.
If three sides of a triangle are equal in length to the same sides of another triangle, then the two triangles are incongruent.
If three pairs of corresponding angles in two triangles are equal, then the triangles are similar.
Name the type of angle (e.g. acute/obtuse etc) based on it's size:
30°
47°
90°
91°
191°
360°
180°
Using Pythagoras' theorem for right-angled triangles, calculate the length of x:
Challenge problem
Using the figure below, show that the sum of the three angles in a triangle is 180
${}^{\circ}$ . Line
$DE$ is parallel to
$BC$ .
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