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Form 5 Similar Triangles
1
Similar Triangles
Core
3.4.1 Symmetry and Congruency
• Understand and know when shapes are similar.
• Understand and use AAA, the common ratio property of sides, and, two common ratios and the included angle to prove similarity of triangles.
• Appreciate that all congruent shapes are similar but similar shapes are not necessarily congruent.
• Appreciate that any two circles and any two squares
• are mathematically similar, whereas, in general, two rectangles are not.
Extension (2A)
• Understand and use the relationship between lengths, areas and volumes of similar shapes.
SEC Syllabus (2015): Mathematics
Section 1 Proving Similar Triangles
• Similar Triangles are those figures with the same shape but one is an enlargement of the other.
• When figures are similar o Corresponding angles are equal o the ratios of all corresponding lengths are equal
There are three conditions how to prove that two triangles are similar
a) A.A.A. (the three angles of one triangle are equal to the angles of the other triangle) b) R.R.R. (the three pairs of corresponding sides are in the same ratio) c) R.A.R. (two pairs of corresponding sides are in the same ratio and the included angles
are also equal)
Note: Remember that when two triangles are SIMILAR, one triangle is always LARGER than the other.
Form 5 Similar Triangles
2
Example 1
In the given diagram prove that triangles ABC and PQR are SIMILAR.
To prove: Triangles ABC and PQR are SIMILAR
Proof:
∠A = ∠P = 30 (given)
∠R = ∠C = 100 (given)
∠Q = ∠B (remaining angles of triangles are equal)
Therefore, triangles ABC, PQR are SIMILAR by A.A.A.
A.A.A. is the most common condition used to prove that two triangles are SIMILAR.
P
Q R
A
B C
100°
100°
30° 30°
Form 5 Similar Triangles
3
Example 2
To prove: Triangles ABC and PQR are SIMILAR
Proof:
𝐵𝐶𝑅𝑄
=64=32
𝐴𝐶𝑃𝑅
=96=32
𝐴𝐵𝑃𝑄
=128=32
Therefore, triangles ABC and PQR are SIMILAR by R.R.R.
• The 3 pairs of corresponding sides are in the same ratio. • This ratio represents the SCALE FACTOR of ENLARGEMENT.
Example 3
To prove: Triangles ABC and PQR are SIMILAR
Proof: In triangles ABC and PQR
𝐵𝐶𝑅𝑄
=96=32
𝐴𝐶𝑃𝑅
=1510
=32
∠ C = ∠ R = 100 (given)
Therefore triangles ABC, PQR are SIMILAR (R.A.R)
Support Exercise Handout
P
Q R
A
B C
6 8
4
6
9 12
P
Q R
A
B C
100°
100°
10
15
6
9
Form 5 Similar Triangles
4
Section 2 Finding missing sides of similar triangles
Considering triangles APQ and ABC
Angle A (Common)
Angles APQ = ABC (Corresponding angles)
Angles AQP = ACB (Corresponding angles)
Therefore, triangles APQ and ABC have equal angles (A.A.A.) and hence they are SIMILAR.
Hence,
𝑎𝑏=𝑐𝑑
𝑚𝑛=
𝑎𝑎 + 𝑏
=𝑐
𝑐 + 𝑑
A
B C
P Q
a
b
c
d
m
n
Form 5 Similar Triangles
5
Example 1
Find x and y:
Triangles APQ and ABC are SIMILAR as proved earlier.
Finding x:
!"!= !
! (Cross Multiplication)
10x = 12
x = 12/10
x = 1.2
𝑦15
=10
10 + 2
!!"= !"
!" (Cross Multiplication)
12y = 10 ╳ 15
12y = 150
y = 150/12
y = 12.5
A
B C
P Q
10
2
6
x
y
15
Form 5 Similar Triangles
6
Example 2
Find x and y:
610
=12𝑥
𝑎!𝑎!
=𝑏!𝑏!
6x = 120
x = 120/6
x = 20
610
=12𝑥
𝑎!𝑎!
=𝑐!𝑐!
6y = 90
y = 90/6
y = 15
Support Exercise Handout
12
6
9
10
x y
a1
a2
b1
b2
c1
c2
Form 5 Similar Triangles
7
Section 3 Similarity involving areas and volume (2a)
• When figures are similar, the ratio of their areas is equal to the ratio of the square of corresponding sides.
• When solid objects are similar, the ratio of their volume is equal to the ratio of the cubes of corresponding sides.
Note: If you are given the ratio of the areas of two similar figures, you have to find the square root to obtain the ratio of the sides.
Note: If you are given the ratio of the volumes of two similar objects, you have to find the cubre root to obtain the ratio of the sides.
Example 1: Triangles ABC and PQR are similar, with angle A = angle P and angle C = angle R. If AC = 4cm, PR = 3 cm and area of triangle PQR is 4.5cm2, find the area of triangle ABC.
AC and PR are corresponding sides and !"!"= !
!
Therefore, !"#! !" !"#$%&'( !"#!!"# !" !"#$%&'( !"#
= !"!
Hence, !!.!= !"
!
𝑥 = 16 × 4.5
9
x = 8
Therefore, area of triangle ABC = 8 cm2
Support Exercise: Handout
4.5 cm2
3 cm P
Q
R
x cm2
4 cm A
B
C