Similar triangles
• If I take a triangle that is
3cm x 4cm x 5cm and
hold it in front of a light.
• I can project a much
larger image on a distant
wall
• 3m x 4m x 5m
• These triangle are similar,
so their measurements
are proportional
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Similar triangles
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If I move the projector closer to the wall
.
the image on the wall shrinks
But it will still be proportional
15cm x 20 cm x 25 cm
The angles of the two triangles are equal
If I cut the base of the large triangle in
half with a line parallel to the left side
The right side of the old triangle will
create a new similar triangle
7.5cm x 10cm x 12.5cm
Each of these measurement is ½ the
previous measurements
The ratio between the two triangles is 1:2
Another way to express the ratio is ½
Similar triangles
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We can simplify many seemingly difficult
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questions simply by discovering similarity
of triangles
When we know the ratio or recognize it
easily we have solved 90% of the
problem.
A 6ft man casts 5 ft shadow next to a
flagpole that is casting a 20 foot shadow.
How tall is the flagpole ?
6/5 equals x/20
Two easy ways to solve
I multiplied 5 x 4 to get 20
Do the same for six (6 x 4 = 24)
Another method 6
x
5

20
 6  20  5  24
?
6
20
5
Similar triangles
Questions from Triand MCAS test
Similar triangles
Questions from Triand MCAS test