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CHAPTER 21
Triangles
Types of Triangles

Triangles with THREE ACUTE ANGLES are called
ACUTE ANGLED triangles.

Triangles with ONE OBTUSE ANGLE are called
OBTUSE ANGLED triangles.

The sum of the INTERIOR ANGLES of a triangle is 180°
92°+37°=129°
z = 180° - 129°
z = 51°
92°
37°
z
Exterior Angle of a Triangle

The EXTERIOR ANGLES of a triangle is always equal to
the sum of the two opposite INTERIOR ANGLES.
x = 46° + 73°
x = 119°
92°
37°
x
NOTE
∆ means triangle
ext means EXTERIOR ANGLE
means Angle
int means INTERIOR ANGLE
supp means SUPPLEMENTARY ANGLE
Special Triangles
1. Scalene
a) All sides different lengths
b) All angles different sizes
c) No axes of symmetry
2. Isosceles
a) Two sides the same length
b) Two angles the same size
c) One axis of symmetry
3. Equitlateral
a) Three sides the same length
b) Three angles the same size (60°)
c) Three axes of symmetry
d) Order of rotational symmetry = 3
Drawing Triangles
If you are just given the lengths of the 3 sides of the triangle
you must use a pair of compasses and a ruler.
If you are given the length of two sides and the size of the
angle between them you must use a protractor and a ruler.
Remember : always draw in pencil
The PERIMETER of a triangle is the total length of its 3 sides.
Area of a Triangle

The area of a TRIANGLE is calculated using the formula:
Area = ½ x base x perpendicular height
this is sometimes written as:
Area = base x perpendicular height
2
A=½xbxh
A=½x8x6
A = ½ x 48
A = 24cm2
6cm
8cm