Triangles are all around us!
Let's learn about them.

★ TRIANGLES ★

Triangles are strong, stable and very interesting!
A triangle is a closed figure with 3 sides, 3 vertices (corners) and 3 angles.
1. PARTS OF A TRIANGLE

A triangle has 3 important parts.

A B C Angle A Angle B Angle C Side AB Side AC Side BC

Vertices (Corners):

A, B, C

Sides:

AB, BC, CA

Angles:

∠A, ∠B, ∠C

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Remember: In any triangle, the sum of all three angles is always 180°.
2. TYPES OF TRIANGLES BY SIDES

Triangles can be named based on the length of their sides.

a) Equilateral Triangle

All three sides are equal.

All sides equal

All angles equal (60° each)

b) Isosceles Triangle

Two sides are equal.

Two sides equal

Two angles equal

c) Scalene Triangle

All three sides are different.

No sides equal

All angles different

3. TYPES OF TRIANGLES BY ANGLES

Triangles can be named based on their angles.

a) Acute Triangle

All three angles are less than 90°.

∠A, ∠B, ∠C < 90°

b) Right Triangle

One angle is exactly 90°.

One angle = 90°

c) Obtuse Triangle

One angle is greater than 90°.

One angle > 90°

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Note: A triangle can be right angled or obtuse angled, but never both at the same time.
4. SPECIAL TRIANGLES
a) Equilateral Triangle

All sides equal and all angles are 60°.

60° 60° 60°
b) Isosceles Right Triangle

Two sides equal and one angle is 90°.

90°
c) Scalene Right Triangle

All sides different and one angle is 90°.

90°
There are many other types also, but these are some important ones!
5. TRIANGLES IN REAL LIFE

Triangles are used in many places because they are strong and do not change shape easily.

Bridges
Pizza slice
! Road signs
Hanger
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Look around you: You will find triangles in buildings, towers, toys, tools and many other things!
6. CLASSIFICATION AT A GLANCE
By Sides By Angles
Equilateral
(3 equal sides)
 Acute
(All angles < 90°)
Isosceles
(2 equal sides)
 Right
(One angle = 90°)
Scalene
(No equal sides)
 Obtuse
(One angle > 90°)
PROPERTIES OF A TRIANGLE
  • ★ A triangle has 3 sides.
  • ★ A triangle has 3 vertices (corners).
  • ★ A triangle has 3 angles.
  • ★ The sum of all three angles is 180°.
  • ★ The longest side is opposite the largest angle.
A B C
∠A + ∠B + ∠C = 180°
PERIMETER OF A TRIANGLE

Perimeter is the total length of the sides.

A B C a b c
Perimeter = a + b + c
AREA OF A TRIANGLE

Area is the space inside the triangle.

A B C h b
Area = ½ × base × height
PRACTICE TIME!
A. Name the type of triangle by sides.
  • 1. _____
  • 2. _____
  • 3. _____
B. Name the type of triangle by angles.
  • 1. _____
  • 2. _____
  • 3. _____
C. Fill in the blanks.
  1. A triangle has _____ sides.
  2. A triangle has _____ angles.
  3. The sum of all angles of a triangle is _____°.
  4. A right triangle has _____ angle equal to 90°.
  5. An equilateral triangle has _____ equal sides.
REMEMBER!
  • ★ Triangles are simple but very important.
  • ★ They are strong, stable and useful in many things.
  • ★ Understand the types and properties, and you will be a Triangle Expert!