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.
The theorem of pythagoras
If
ABC is right-angled (
) then
Converse: If
, then
ABC is right-angled (
).
In the following figure, determine if the two triangles are congruent, then use the result to help you find the unknown letters.
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.
Sides
Name
5
pentagon
6
hexagon
7
heptagon
8
octagon
10
decagon
15
pentadecagon
Table of some polygons and their number of sides.
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:
where
is the number of sides and
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
and
Right angle: An angle measuring
Obtuse angle: An angle
and
Straight angle: An angle measuring
Reflex angle: An angle
and
Revolution: An angle measuring
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
Exercises
Find all the pairs of parallel lines in the following figures, giving reasons in each case.
Find angles
,
,
and
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
. Line
is parallel to
.
Wayne and Dennis like to ride the bike path from Riverside Park to the beach. Dennis’s speed is seven miles per hour faster than Wayne’s speed, so it takes Wayne 2 hours to ride to the beach while it takes Dennis 1.5 hours for the ride. Find the speed of both bikers.
from theory: distance [miles] = speed [mph] × time [hours]
info #1
speed_Dennis × 1.5 = speed_Wayne × 2
=> speed_Wayne = 0.75 × speed_Dennis (i)
info #2
speed_Dennis = speed_Wayne + 7 [mph] (ii)
use (i) in (ii) => [...]
speed_Dennis = 28 mph
speed_Wayne = 21 mph
George
Let W be Wayne's speed in miles per hour and D be Dennis's speed in miles per hour. We know that W + 7 = D and W * 2 = D * 1.5.
Substituting the first equation into the second:
W * 2 = (W + 7) * 1.5
W * 2 = W * 1.5 + 7 * 1.5
0.5 * W = 7 * 1.5
W = 7 * 3 or 21
W is 21
D = W + 7
D = 21 + 7
D = 28
Salma
Devon is 32 32 years older than his son, Milan. The sum of both their ages is 54 54. Using the variables d d and m m to represent the ages of Devon and Milan, respectively, write a system of equations to describe this situation. Enter the equations below, separated by a comma.
please why is it that the 0is in the place of ten thousand
Grace
Send the example to me here and let me see
Stephen
A meditation garden is in the shape of a right triangle, with one leg 7 feet. The length of the hypotenuse is one more than the length of one of the other legs. Find the lengths of the hypotenuse and the other leg
however, may I ask you some questions about Algarba?
Amoon
hi
Enock
what the last part of the problem mean?
Roger
The Jones family took a 15 mile canoe ride down the Indian River in three hours. After lunch, the return trip back up the river took five hours. Find the rate, in mph, of the canoe in still water and the rate of the current.
Shakir works at a computer store. His weekly pay will be either a fixed amount, $925, or $500 plus 12% of his total sales. How much should his total sales be for his variable pay option to exceed the fixed amount of $925.
I'm guessing, but it's somewhere around $4335.00 I think
Lewis
12% of sales will need to exceed 925 - 500, or 425 to exceed fixed amount option. What amount of sales does that equal? 425 ÷ (12÷100) = 3541.67. So the answer is sales greater than 3541.67.
Check:
Sales = 3542
Commission 12%=425.04
Pay = 500 + 425.04 = 925.04.
925.04 > 925.00
Munster
difference between rational and irrational numbers
Jazmine trained for 3 hours on Saturday. She ran 8 miles and then biked 24 miles. Her biking speed is 4 mph faster than her running speed. What is her running speed?