Gesture

Before entering junior high school you have recognize a wide range of plane, such as trian-gles. You also know how to grouping plane was based shapes, elements, and atributes. To refresh your memory, look at the picture on the side. Try to find other objects that triangular surface!

1. Focus Motivation the Lesson How many triangle can you find at the picture below?

5-minutes

check

Do you know

risoles? What

shape is it?

Do you know

traffic sign?

What shape is

it?

Basic Competence (ies)

Identifying triangle types based on size of sides and its angles

What will you

learn??

Determine types

of triangle based

on size of sides.

Determine types

of triangle based

on size of angles.

TYPES OF TRIANGLE

Why it is

important?

You can learn to determine types of triangle based on size of sides You can learn to determine types of triangle based on size of angles

Page 1 “Books can be dangerous. The best ones should be

labeled ‘This could change your life’.” (Helen Exley)

Definition of Triangle

In elementary school you learned

that from the three points that do

not lie on a straight line, can be

made of the so called triangular

plane. The picture on the left shows

three points A, B, and C are not

aligned. If the three points

are connected, will form a triangle ABC. Usually triangles

denoted by "Δ" , written Δ ABC be a triangle ABC. The

elements contained in Δ ABC is

a. Points A, B, and C is called the vertex.

b. AB, BC and CA are called the sides of the triangle.

Keyword: triangle,types, size, sides, angles.

3

A

B

C

The triangle which all side have same length is called a equilateral triangle.

The triangle which two equal sides have same length is called isosceles triangle.

The triangle which all side have un-equal length called scalene triangle.

Page 2 “A book is like a garden carried in the pocket.”

(Chinese Proverb)

1. Types of triangle based on size of side

Look at the picture below!

A. Use a ruler to measure the length of the sides of ΔABC

B. Are there congruent sides? If there are, how many sides that are congruent?

C. By looking at the length of the sides, what is ΔABC plane called? Explain!

D. The same question with (a) to (c) for ΔDEF.

E. The same question with (a) to (c) for ΔPQR.

Relation to the Real World

Look at the picture (a), what shape is the towering buildings?

At (b), what shape is the motor boat?

Look at the image (c), what shaped is the fishing boat?

2. PRACTICE /

APPLY

Using Cooperative

Groups

Have five of students

discuss this problem

Given triangle that have

AB= 6 cm, BC=5 cm

and AC=5 cm.

a. Mention the type of

triangle! Why?

b. Can you classify

triangle by looking at

the length of the sides?

Source: Dit. PSMP,

E

Source: BSE Mathematic Grade 7,2008

Page 3 “The beautiful thing about learning is that no

one can take it away from you.”(B.B. King)

Think andDiscuss!

Look at the picture on the side!

How many equilateral triangle with

sides one unit?

How many equilateral triangle with

sides two unit?

Look at the KLM isosceles triangle on left side!

Called top angle. and called

base angle.

K L M

MuLuML .

Checking For

Understanding

Think and discuss

are designed to

help you assess

students’ under-

standing through

reading, writing,

and speaking.

2. Types of triangle based on size of angle Remember to return the material in the previous chapter about the

types of of angles. In general there are three types of angles,

that is:

1. Acute angle (00< x < 900)

2. Obtuse angle (900< x < 1800)

3. Reflex angle (1800< x < 3600)

Related to that, when viewed from large angle, there are three

types of triangles as follows.

(i) Acute triangle

Acute triangle is a triangle that third of

angle is acute angle, so the size of angles in

the triangle are about 0o and 90o. At the

picture (i) on right side, the third angle of the

ΔABC is acute angle.

(ii) Obtuse triangle

Obtuse triangle is a triangle which has an

obtuse angle. At ΔABC on left side is

obtuse angle

(iii) Right triangle

Right triangle is a triangle which has a right

angle (magnitude 90o). At the picture (iii) on right

side, right-angled ΔABC at vertex C .

(Grow Your

Creativity)

Make a right

triangle,

isosceles

triangle, and

equilateral

triangle of cardboard.

Show the properties of

each of these triangle.

Do it in front of the

class.

ABC

Gesture

So

urc

e: B

SE

Mat

hem

atic

Gra

de

7,2

008

Source: BSE Mathematic Grade 7,2008

Page 4 “They success, because they think they

can” (Virgil)

Relation to the real world

Picture (a), what shaped is sail of boat?

Picture (b) , what shape is wood at the terrace house picture?

Look at the picture !

Construct two right-

angled triangle

Try to arrange two right-angled triangle whose shape and size as a geometry

plane by attaching the other side of the same length!

Draw each geometry plane which you have!

How much different geometry plane which you have?

Mention the names of geometry plane which you have!

Crittical

Thinks

Look at the picture!

In the picture above, the line

AD // BC and PQ // SR.

a. Mention couples angle

equal!

b. If large PSR=65o,specify

large angle other. Determine

the type of each of the

corners too!

Close

Guide students to

summarize the lesson.

Exercise!

1. from the triangles on the right, which is classified

a.Isosceles triangle;

b.Equilateral triangle;

c.Scalene triangle;

d.Acute triangle;

e.Right triangle;

f.Obtuse triangle;

2. On checkered paper draw triangle ABC with A (2, 1), B(4,1), and C (3, 4). what kind of

triangle ABC? Give your reasons!

Source: BSE Mathematic Grade 7,2008

Source: BSE Mathematic Grade 7,2008