Lesson Notes By Weeks and Term - Junior Secondary School 2

ANGLES IN POLYGON

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SUBJECT: MATHEMATICS

CLASS:  JSS 2

DATE:

TERM: 3rd TERM

REFERENCE

  • WABP ESSENTIAL MATHEMATICS FOR JSS BK 2 BY A.J.S. OLUWASANMI
  • NEW GENERAL MATHEMATICS BY J.B. CHANNON & ETAL

 

 
WEEK TWO

TOPIC: ANGLES IN POLYGON

CONTENT:    (i) Sum of interior angles of a polygon

        (ii) Sum of exterior angles of a polygon

 

DEFINITION OF A POLYGON

A polygon is any close plane figure with straight side. A regular polygon has all sides and angles equal. 

Polygon are named according to the number of sides they have. Examples are:

Triangle         a 3- sided polygon

Quadrilateral        a 4- sided polygon

Pentagon        a 5- sided polygon

Hexagon        a 6- sided polygon

Heptagon        a 7- sided polygon

Octagon        a 8- sided polygon

Nonagon        a 9- sided polygon

Decagon        a 10- sided polygon

 

The diagrams below represent some common polygons. 




Triangle             Quadrilateral            Pentagon        Hexagon

(3 sided)            (4- sided)            (5- sided)        (6- sided)

 

Reference

NGM Book 2

Essential Mathematics for junior secondary school Book 2, chapter 9, pages 87 – 88

 

Sum of Interior Angles of a Polygon

The angles inside a polygon are called its interior angles as shown in the figure below:



                            e = exterior angle 

 

                    i = interior angle 









The number of triangles depends on the number of sides of the polygon. For a polygon with ‘n’ sides there will be (n-2) triangles. The sum of angles of a triangle is 1800.

Alternatively, since 1800 = (n-2) x 2 x 90

= 2(n-2) 90

= (2n-4) 90

 

Thus, the sum of the angles of an n-sided polygon can be represented as (n-2) 1800 or (2n-4) 900

The table below shows the sum of interior angles of a regular polygon of a 3 sided polygon up to a sided polygon.

Polygon

No. of Sides

No. of Triangles 

(n-2)

Sum of interior Angles (n-2) 1800

Triangle 

3

3 – 2 = 1

1 x 1800 = 180

Quadrilateral

4

4 – 2 = 2

2 x 1800 = 3600

Pentagon

5

5 – 2 = 3

3 x 1800 = 5400

Hexagon

6

6 – 2 = 4

4 x 1800 = 7200

Heptagon

7

7 – 2 = 5

5 x 1800 = 9000

Octagon

8

8 – 2 = 6

6 x 1800 = 10800

Nonagon

9

9 – 2 = 7

7 x 1800 = 12600

Decagon

10

10 – 2 = 8

8 x 1800 = 14400

Worked Examples: 

  1. Calculate the size of exterior angle of a regular nonagon (9 sides)
  2. Calculate the size of exterior angle of a regular hexagon (6 sides)

 

Answer to the evaluation questions 

  1. Sum of exterior of a polygon = 3600. Number of sides of a nonagon = 9

Size of each exterior angle = sum of exterior angles/number of sides.

                = 360°9

                = 40o

  1. Sum of exterior angles = 360o

Hexagon has 6 sides.

Size of each exterior angle = sum of exterior anlgesno of sides

                    = 360°6=60o

GENERAL EVALUATION

  1. The interior angles of a triangle add up to …………………………
  2. The interior angles of a quadrilateral add up to ………………………
  3. The sum of the interior angles of a regular polygon is 1080o. How many sides has the polygon?

 

REVISION QUESTION

  1. Calculate the number of sides of each of a regular polygon whose interior angle is 162o
  2. The sum of the 3 angles of a hexagon is 345o. If the other angles are equal. Find the sizes of each of the angle.

 

READING ASSIGNMENT

Essential Mathematics for junior secondary school Book 2, Chapter 19, page 252 - 255

Exercise 19.5 No 1 page 255

 

WEEKEND ASSIGNMENT

  1. The sum of interior for angle of a regular pentagon is   A. 240o      B. 720o    C. 540o     D. 640o
  2. Calculate the size of each of exterior angle of a regular hexagon.   A. 60o    B. 30o     C. 45o     D. 125o
  3. The size of each angle of a regular octagon will be ____ A. 95o     B. 75o     C. 105o     D. 135o
  4. How many sides has a polygon if the sum of interior angles of that polygon gives 3240o?     A. 18o      B. 15o      C. 17o     D. 20o
  5. Calculate the size of each exterior angles of a pentagon     A. 30o     B, 72o      C. 60o     D. 90o

 

THEORY

  1. Calculate A. The total internal angels of an octagon B. The size of each angle of a regular octagon
  2. Calculate the 
  1. Exterior angle
  2. The number of sides of a regular polygon with an interior angle of 72o





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