JSS2 Mathematics Third Term: Angles in a Polygon

Topic: ANGLES IN A POLYGON

Angles between lines

If a line is split into 2 and you know one angle you can always find the other one.

Example: If we know one angle is 45° what is angle “a” ?

Angle a is 180° − 45° = 135°

This method can be used for several angles on one side of a straight line.

Example: What is angle “b” ?

Angle b is 180° less the sum of the other angles.

Sum of known angles = 45° + 39° + 24°
Sum of known angles = 108°

Angle b = 180° − 108°
Angle b = 72°

Vertically opposite angles are equal

Vertically Opposite Angles are the angles opposite each other when two lines cross

“Vertical” in this case means they share the same Vertex (or corner point), not the usual meaning ofup-down.
In this example, a° and b° are vertically opposite angles. The interesting thing here is that vertically opposite angles are equal:

a⁰ = b⁰

Angles in a triangle

In a triangle, the three interiorangles always add to 180°:

A + B + C = 180°

Example: Find the Missing Angle “C”

 

Start with:A + B + C = 180°

Read more below:

ANGLES IN A POLYGON