Transcript No Slide Title

Congruent and
similar triangles
You are going to draw a triangle onto card
or paper
You will need a ruler and compasses to
construct it
Using a ruler and compasses only,
this is how to construct a triangle
of sides 8 cm, 7 cm & 4 cm
Draw a line of 8 cm
Put the point of
compasses here &
draw an arcs of
radius 7 cm
7 cm
4 cm
Put the
point here
& draw an
arc of
radius 4 cm
8 cm
On card, construct 2 triangles with sides 8 cm, 7cm & 4 cm
Cut them out
Your two triangles are the same shape and size
They are congruent
This is how to construct an equilateral triangle
using a ruler and compasses
Put the point of
compasses here &
draw two arcs of
equal radius
Keep the radius the
same, put point here &
draw another arc
On card, construct 2 equilateral triangles
of different sizes. Cut them out.
Your two triangles are the same
SHAPE
They are similar
They are different
SIZES
On card, draw 2
different triangles with
base angles 75º and 55º
Use a ruler
and
protractor
55º
75º
Cut them out
Measure the third angles
(They should both be 50º)
These two triangle have
the same shape but
are different sizes
They are
similar
triangles
They have the same angles
Which triangle is not similar to the others?
Answer
These two triangles are both
equilateral. Are they similar?
Answer
YES
Problems involving
triangles can be solved by
scale drawing
Scale drawings are similar
to the real thing
North
Island
The map shows the
journey of a ship.
The ship leaves the Port,
sails 30 miles North, then
20 miles East.
MAP
N
I
Make a scale
drawing & produce a
similar diagram.
Port
How far is the Island
from the port ?
DRAWING
?
P
End