Grade 9
Geometry
Grade 8 revision:
 Labeling angles:
y
ABˆ C is an angle. We could also write it as ____________ or
___________ .
AB and BC are the _____________ of the angle.
B is the ___________________ of the angle.
 Classifying angles:
Name
Acute angle
Right angle
Obtuse angle
Straight line
Reflex angle
Revolution
Diagram
Description
 Angle pairs:
If two angles add up to _______ they are called COMPLEMENTARY angles.
If two angles add up to _______ they are called SUPPLEMENTARY angles.
 Adjacent angles:
Two angles are adjacent if they satisfy the following:
1. They have a common __________
2. They have a common __________ and they are on either side of the common
________ .
B̂1 and B̂ 2 are adjacent angles.
Geometry methods:

Angles on a straight line add up to 180o / are supplementary.
( ' s on a str. line)

When two lines intersect, the angles that lie opposite to each other are equal.
(vert. opp ' s )

Angles around a point add up to 360o.
( ' s around a pt)

Parallel lines:
-
If parallel lines are cut by a transversal, the corresponding
angles are equal. The two angles form an F.
(corresp. ' s , AB//CD)
-
If parallel lines are cut by a transversal, the alternate angles
are equal. The two angles form a Z.
(alt. ' s , AB//CD)
-
If parallel lines are cut by a transversal, the co-interior
angles are supplementary. The two angles form a U.
(co-int. ' s , AB//CD)
Triangles:
Draw a triangle Draw a triangle Draw a triangle Draw a triangle Draw a triangle Draw a triangle
with acute
with a right
with one obtuse with all sides
with two sides
with all sides
angles only.
angle.
angle.
different
equal in length. equal length.
lengths.
Name of Δ :
Name of Δ :
Name of Δ :
Name of Δ :
Name of Δ :
Properties of triangles:
 When any two sides of a triangle are equal in length, then the angles
opposite these sides are equal in size.
( ' s opp. = sides)
Converse: When two angles in a triangle are equal, the sides opposite
those angles are equal in length.
(sides opp. = ' s )
 The sum of the interior angles of any triangle is 180o.
( ' s sum Δ )
 The exterior angle of a triangle is equal to the sum of the interior opposite angles.
(ext.  of Δ = sum int opp . ' s )
Name of Δ :