PolygonsPolygonsPolygonsPolygons

A many sided figureA many sided figure

The cross section of a brilliant-cut diamond forms a pentagon. The most beautiful

and valuable diamonds have

precisely cut angles that maximize the

amount of light they reflect.

A pentagon is a type of polygon.

Prefixes are used to name different types

of polygons.

Polygon – a closed plane figure formed by three or more segments.Regular polygon – a polygon with congruent sides and angles.

Prefixes used to name polygons: tri-, quad-, penta-, hexa-, hepta-, octa-, nona-, deca-

Polygons are named (classified) based on the number of sides.

PolygonsProperties of polygons, interior angles of

polygons including triangles, quadrilaterals, pentagons, heptagons, octagons, nonagons, and decagons.

Properties of TrianglesTriangle – 3-sided polygon

The sum of the angles in any triangle is 180° (triangle sum theorem)

The formula we use to find the sum of the interior angles of any polygon comes from

the number of triangles in a figure

First remember that the sum of the interior angles of a polygon is given by the formula 180(n-2).A polygon is called a

REGULAR when all the sides are congruent and

all the angles are congruent.

The picture shown to the left is that of a Regular

Pentagon. We know that to find the sum of its

interior angles we substitute n = 5 in the

formula and get:180(5 -2) = 180(3) = 540°

Regular triangles - Equilateral

All sides are the same length (congruent) and all interior angles are the same size

(congruent).To find the measure of the

interior angles, we know that the sum of all the angles equal 180°, and there are

three angles.So, the measure of the

interior angles of an equilateral triangle is 60°.

Quadrilaterals – squaresAll sides are the same length (congruent) and all interior angles are the same size

(congruent)To find the measure of the

interior angles, we know that the sum of the angles equal

360°, and there are four angles, so the measure of the

interior angles are 90°.

Pentagon – a 5-sided polygon

To find the sum of the interior angles of a

pentagon, we divide the pentagon into triangles. There are three triangles and because the sum of each triangle is 180° we

get 540°, so the measure of the interior

angles of a regular pentagon is 540°

Hexagon – a 6-sided polygon

To find the sum of the interior angles of a

hexagon we divide the hexagon into triangles. There are four triangles and because the sum of

the angles in a triangle is 180°, we get 720°, so the

measure of the interior angles of a regular hexagon is 720°.

Octagon – an 8-sided polygonAll sides are the same length (congruent) and all interior angles are the same size

(congruent).What is the sum of the angles in a regular

octagon?

Nonagon – a 9-sided polygon

All sides are the same length (congruent)

and all interior angles are the same size

(congruent).

What is the sum of the interior angles of a regular nonagon?

Decagon – a 10-sided polygon

All sides are the same length (congruent) and

all interior angles are the same size (congruent).

What is the sum of the interior angles of a regular decagon?

Using the pentagon example, we can come up with a formula that

works for all polygons.

Notice that a pentagon has 5

sides, and that you can form 3 triangles by connecting the

vertices. That’s 2 less than the number of sides. If we represent the number of sides of a polygon

as n, then the number of

triangles you can form is (n-2). Since each triangle contains

180°, that gives us the formula:

sum of interior angles =

180(n-2)

Warning !• Look at the pentagon to

the right. Do angle E and angle B look like they have the same measures? You’re right---they don’t. This pentagon is not a regular pentagon.

• If the angles of a polygon do not all have the same measure, then we can’t find the measure of any one of the angles just by knowing their sum.

Using the Formula

Example 1: Find the number of degrees in the sum of the interior angles of an octagon.

An octagon has 8 sides. So n = 8. Using our formula, that gives us 180(8-2) = 180(6) =

1080°

Example 2: How many sides does a polygon have if the sum of its interior

angles is 720°?

Since, this time, we know the number of degrees, we set the formula equal to 720°,

and solve for n.180(n-2) = 720 set the formula = 720°n - 2 = 4 divide both sides by 180

n = 6 add 2 to both sides

Names of Polygons

Triangle 3 sidesQuadrilateral 4 sidesPentagon 5 sidesHexagon 6 sidesHeptagon or Septagon 7 sidesOctagon 8 sidesNonagon or Novagon 9 sidesDecagon 10 sides

Practice with Sum of Interior Angles

1) The sum of the interior angles of a hexagon.

a) 360°b) 540°c) 720°

2) How many degrees are there in the sum of the interior angles of a 9-sided polygon?

a) 1080°b) 1260°c) 1620°

3) If the sum of the interior angles of a polygon equals 900°, how many sides does the polygon have?

a) 7b) 9c) 10

4) How many sides does a polygon have if the sum of its interior angles is 2160°?

a) 14b) 16c) 18

5) What is the name of a polygon if the sum of its interior angles equals 1440°?

a) octagonb) decagonc) pentagon

Special QuadrilateralsSpecial QuadrilateralsSpecial QuadrilateralsSpecial Quadrilaterals

4-sided figures4-sided figures

Quadrilaterals with certain properties are given additional names.

A square has 4 congruent sides and 4

right angles.

A rectangle has 4 right angles.

A parallelogram has 2 pairs of parallel

sides.

A rhombus has 4

congruent sides.

A kite has 2 sets of adjacent sides

that are the same length

(congruent) and one set of

opposite angles that are

congruent.