6.8 Enlargements
scale factor = 3
(The triangle is three times bigger.)
3x
x
(Use a compass and a ruler for measuring these.)
‘Centre of enlargement’
6.9 Similar Triangles (Theorem 13)
A
A
B0
scale factor = 2
(The triangle is twice bigger.)
C0
B
C
B
Two triangles are said to be ‘similar’ if they have three common angles. The lengths of the sides
can be different, but you will find that the ratios of the sides are proportional.
A
B0
A
C0
C0
A
B0
C0
A
B0
B0
C0
B
B
C
B
B
C
C
C
These triangles 4ABC and 4AB 0 C 0 are all ‘similar’ to one another. [Prove it to yourself by
measuring the lengths of the sides with a ruler. You will see that they are always proportional.]
Sometimes it can be hard to spot this. A tell-tale sign, is to see if there is a common angle. If you
can see that, this means that one of the conditions is there for the triangles to be ‘similar’.
‘Congruent’ triangles are also ‘similar’.
Scale factor = 1.
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