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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