1. TRIANGLE
Segitiga

2. Triangle Around Us

3. 6.1 Indentify the property of triangle based on their sides and angles
BASE COMPETENCE
6.1 Indentify the property of triangle based on their sides and angles
6.2 Drawing a triangle, altitude, bisector, Median and axis on triangle
6.3 Count the perimeter and the area of triangle, and how to use in problem solving

4. Definition of triangle
Triangle is a plane figure bounded by three non collinear lines and forming three inner (interior) angles. A b
Side BC in front of angle A can be written as side a
Side AC in front of angle B can be written as side b
Side AB in front of angle C can be written as side c c C a B
Triangle ABC
Parts of Triangle:
Points A, B adn C are called vertex
AB,BC and AC are sides

5. Angles in a Triangles

6. 1. Inner (Interior) Angles of Triangles (Sudut Dalam Segitiga
The sum of inner angle is 180o C
∠A + ∠B + ∠C = 180o A B

7. EXERCISES-1

8. 2

9. 3

10. 4 5 6

11. 2. Exterior Angles of Triangle
In Exercise 1 you were dealing with the angles inside a triangle, called interior angles. In this section we will look at the angles outside a triangle, called exterior angles.
If one side of a triangle is extended, the angle between this extension and the
triangle is called an exterior angle.

12. Investigate the exterior angles of trianglesC
60o
48o B A
72o E D

13. Conclusion

14. 1 2 3

15. A. Base on the size interior angle
THE TYPES OF TRIANGLE
A. Base on the size interior angle
Acute Triangle (Segitiga Lancip)
All the interior angles are acute angles

16. 2. Obtuse Triangle (Segitiga Tumpul)
one of the interior angles is obtuse angle

17. 3. Right Triangle (Segitiga Siku-Siku)
one of the interior angles is right angle

18. Exercise Determine the type of triangle bellow if
1. The angle are : 65, 35, 80
2. The angle are : 25, 60, 95
3. The angle are : 54, 56, 70
4. Two angle are : 73, 34,
5. The ratio of angle is 3 : 4 : 5
6. The ratio of angle is 2 : 3 : 4
7. The angle is 6x, 2x + 3, 4x +9

19. determine the value of x and y and the type of triangle

20. B. Base on the lengths of their sides (Berdasarkan panjang sisinya)
THE TYPES OF TRIANGLE
B. Base on the lengths of their sides (Berdasarkan panjang sisinya)
Scalene Triangle (Segitiga Sembarang) C A B
Scalene Triangle is a triangle whose three sides are not equal in length.
AB ≠ BC ≠AC

21. 2. Isosceles Triangle (Segitiga Samakaki)B
Isosceles Triangle is a triangle which the two sides are equal in length
AB = AC

22. 3. Equilateral Triangle (Segitiga Samasisi)C A B
Equilateral Triangle is a triangle whose the three sides are equal in length
AB = AC = BC

23. (Berdasarkan panjang sisi dan besar sudut)
THE TYPES OF TRIANGLE
Base on the lengths of their sides and the measure of its interior angle
(Berdasarkan panjang sisi dan besar sudut)
1. Isosceles Triangle (Segitiga Samakaki)
Obtuse angled-isosceles triangle
(segitiga tumpul samakaki)
Right angled-isosceles triangle
(segitiga siku-siku samakaki
Acute angled-isosceles triangle
(segitiga lancip samakaki)

24. 2. Equilateral Triangle (Segitiga samasisi)
60o
60o
60o
Equilateral triangle
(segitiga samasisi)
It has equal measure of all sides and the measure of every interior angle = 60o

25. 3. Scalene Triangle (Segitiga Sembarang)
Obtuse angled-scalene triangle
(segitiga tumpul sembarang)
Right angled-scalene triangle
(segitiga siku-siku sembarang)
Acute angled-scalene triangle
(segitiga lancip sembarang)

26. Answer the following questions
Is there a right-angled equilateral triangle? (adakah segitiga siku-siku samasisi?)
Is there a obtuse-angled equilateral triangle? (adakah segitiga tumpul samasisi?)
Is there a acute-angled scalene triangle? (adakah segitiga lancip sembarang?)
Is an equilateral triangle always an acute triangle?

27. Properties of Triangle
1. Isosceles Triangle (Segitiga Samakaki)
It has 2 equal sides
It has 2 equal angles
It has one axis of simmetry (mempunyai satu sumbu simetri)
It can fits its frame in 2 ways (dapat menempati bingkainya dengan 2 cara)

28. 1. Isosceles Triangle (Segitiga Samakaki) C
It can fits its frame in 2 ways (dapatmenempati bingkainya dengan 2 cara) A B D
First posisition C B A D
Second posisition

29. Properties of Triangle
2. Equilateral Triangle (Segitiga Samasisi)
Answer the following questions:
How many equal sides are in equilateral triangle?
How many equal angles are in equilateral triangle?
How many axis of simmetry are in equilateral triangle?
In how many ways it can fits its frame?

30. 2. Equilateral Triangle (Segitiga Samasisi)
It has 3 equal sides
It has 3 equal angles
It has 3 axis of simmetry (mempunyai 3 sumbu simetri)
It can fits its frame in 6 ways (dapat menempati bingkainya dengan 6 cara)

31. Properties of Triangle
3. Right Triangle (Segitiga Siku-siku) C
Based on the figure on the left, then:
BC2 = AC2 + AB2 Or
a2 = b2 + c2
hypotenuse
(sisi miring)
kaki / sisi siku-siku
leg / right side A
leg / right side
kaki / sisi siku-siku B
For every right triangle:
the square of its hypotenuse equals the sum of the square of the other sides.
It called Pythagorean Theorem

32. a=17 cm 17 cm ? c=8 b=15 b2 + c2 = a2 Example 1
A triangle has legs measuring 8 cm and 15 cm, what is the length of the hypotenuse??
1. Draw a picture
2. Write down Pythagorean theorem
a=17 cm ?
c=8
b2 + c2 = a2
3. Substitute in what you know
b=15
=a2
= a2
4. Take square root!!
289= a2
17 cm
17= a

33. b2 + c2 = a2 Example 2 b=12 a=20 ? C=16 cm 122+ c2 = 202 144+ c2 = 400
A triangle has hypotenuse measuring of 20 cm and of its legs measuring 12 cm. Find the length of the other legs
1. Draw a picture
2. Write down Pythagorean theorem
b=12
a=20
b2 + c2 = a2
3. Substitute in what you know
C=16 cm ?
122+ c2 = 202
4. Take square root!!
144+ c2 = 400
c = √256 = 16 cm
c2=
c2= 256

34. Problem 1
Based on the following figure, form the equation using Pythagorean Theorem

35. Problem 2
Based on the following figure, find the value of x

36. Problem 3
Based on the following figure, find the value of y

37. Bilangan Tripple Pythagoras
Yaitu: 3 bilangan yang dapat digunakan sebagai sisi-sisi dari suatu segitiga siku-siku
Dasar
Kelipatan
Sisi miring
Dua sisi siku-siku 5 3 4
2,5 10 15
1,5 6 9 2 8 12 13
6,5 26 24 17
8,5 34
7,5 30 16 29 21 20 58 42 40 25 7 50 48 14

38. Perimeter of Triangle (Keliling Segitiga)C
Perimeter of ∆ABC = AB + BC + AC
= c + b + a
= a + b + c
a cm
b cm A
c cm B

39. Problem 1
In an isosceles triangle ABC. AB=BC, if AB = 15 cm and AC = 10 cm. Find the perimeter of triangle ABC
Perimeter = AB + BC + AC =
= 40 cm B
15 cm
15 cm A
10 cm C

40. Problem 2
The perimeter of triangle ABC is 120 cm. If AB:BC:AC=3:4:5, the length of AB is….
Solution:
AB + BC + AC = 120 cm
AB = (3/12) x 120 cm
= 30 cm.

41. Problem 3
The perimeter of triangle ABC is 84 cm. If a : b : c = 5 : 3 : 4, the length of BC is….

42. Problem 4 Observe the following figure.
If the perimeter of triangle PQR on the left = 180 cm, so the length of QR is .... 2x 3x Q 4x R

43. Problem 5 The perimeter of triangle ABC below is … . C 8 cm 17 cm
D 6 cm A B

44. Area of Triangle Area of a triangle = ½ (base x altitude) = ½ b.h C
AB is the base of ∆ABC,
DC is altitude/height of ∆ABC A D B
Area of a triangle = ½ (base x altitude)
= ½ b.h

45. Example: R
If PQ = 10 cm, RS= 12 cm and PR = 13 cm. The area of ∆PQR is…. S Q P
Solution
The area of ∆PQR = ½ x base. height
= ½ x PQ.RS
= ½ x 10 x 12
= 60 cm2

46. Problem 1
The ratio of base and altitude of a triangle is 4 : 5. If the area of the triangle is 90 cm2, then find the base and altitude of the triangle.
Solution:
Area = ½ .b. h
90 = ½ . 4x. 5x
90 = 10x2
9 = x2
3 = x 5x 4x

47. Problem 2 C
Find the area of the green region, if: AB = 20 cm DC = 6 cm DE = 5 cm D A E B
Solution:
Area of green = Area ∆ABC – area ∆ABD
= ½ . AB. CE – ½ .AB. DE
= ½ – ½.20.5 = 60cm2

48. Problem 3 R
Find the area of the blue region, if: PQ = 10 cm RS = 12 cm TU = 4 cm T P U Q S
Solution:
Area of Blue = Area ∆PQR – area ∆PQT
= ½ . PQ. RS – ½ .PQ. TU
= ½ – ½.10.4
= 60 – 20 = 40 cm2

49. Problem 4
The ratio of right sides of a right triangle is 3 : 4. if the area of the triangle is 150 cm2, find the perimeter of the triangle.
Solution:
Area = ½ .b. h
150 = ½ . 3x. 4x
150 = 6x2
25 = x2
5 = x 4x 3x
The right sides are : 15 cm and 20 cm.
Using pythagorean theorem, the length of hypotenuse = 25 cm
Perimeter = 15 cm + 20 cm + 25 cm = 60 cm

50. Base and Altitude of Triangle
1. Right Triangle C
Remember!
Altitude is perpendicular to the base
(tinggi segitiga tegak lurus dengan alas) D A B
AC is the altitude to the base AB
Area of ∆ABC = ½ x AB x AC
AD is the altitude to the base BC
Area of ∆ABC = ½ x AD x BC

51. Example 1 If AB = 8 cm and AC = 6 cm, then find: Length of BC
Area of ∆ABC
Length of AD C D
6 cm A
8 cm B

52. Base and Altitude of Triangle
2. Acute Triangle C
DC is the altitude to the base AB
Area of ∆ABC = ½ x AB x DC E F
AE is the altitude to the base BC
Area of ∆ABC = ½ x BC x AE A D B
FB is the altitude to the base AC
Area of ∆ABC = ½ x AC x FB

53. Example 2: If PQ = 16 cm, RQ = 15 cm and PT = 8 cm.
Find the area of ∆PQR.
Find the length of RS. R T P S Q

54. Base and Altitude of Triangle
3. Obtuse Triangle C
DC is the altitude to the base AB
Area of ∆ABC = ½ x AB x DC E
AE is the altitude to the base BC
Area of ∆ABC = ½ x BC x AE B A D F
FB is the altitude to the base AC
Area of ∆ABC = ½ x AC x FB

55. Example 3 If HD = 9 cm, DE = 7 cm and FH = 12 cm.
Find the area of ∆DEF.
Find the length of DG. F G H D E

56. Problem 4 D
20 cm C
12 cm A B
ABCD is a rectangle (persegi panjang), then find the area of the shaded region.