PolygonsPolygonsA 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 - EquilateralAll 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 PolygonsTriangle 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 Quadrilaterals4-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.

Algebra in GeometryAlgebra in GeometryApplying Geometric PropertiesApplying Geometric Properties

Algebra can be used to solve many problems in geometry. Using variables and algebraic

expressions to represent unknown measures makes solving many problems easier.

Find the sum of interior angles using the formula.

180°(n - 2) = 180°(4 – 2) =180°(2) = 360°Set the sum of the angles equal

to the total.120° + 50° + 80° + x = 360°250° + x = 360°250 – 250 + x = 360 -250

x = 110°

Remember, a regular polygon has congruent sides and

congruent angles.Given the regular pentagon at

the left, what are the measures of the interior angles. (use the formula)

180°(n – 2) =180°(5 – 2) =

180°(3) = 540°# of angles = 5540°/5 = 108°

Each angle in a regular pentagon measures 108°

Using geometry to solve word problems.Remember, draw a picture.

Quadrilateral STUV has angle measures of:

(3x + 15)°(2x + 20)°(4x + 5)°(2x – 10)°, add the angles = 360(3x + 15) + (2x + 20) + (4x + 5)

+ (2x – 10) = 36011x + 30 = 36011x = 330x = 30°

x = 30°, then3x + 15 = 3(30) + 15 = 105°2x + 20 = 2(30) + 20 = 80°4x + 5 = 4(30) + 5 = 125°2x – 10 = 2(30) – 10 = 50°So,105° + 80° + 125° + 50° = 360°

Solve the following:

Figure ABCDEF is a convex polygon with the following angle measures. What is the measure of each angle? (draw a picture)

A = 4xB = 2xC = 3xD = 5x + 10E = 3x – 20F = 2x – 30 Answer »»

(4x) + (2x) + (3x) + (5x + 10) + (3x – 20) + (2x – 30) = 720°19x – 40 = 720°

19x = 720°x = 40°, so

4x = 4(40) = 160°2x = 2(40) = 80°3x = 3(40) = 120°

5x + 10 = 5(40) + 10 = 210°3x – 20 = 3(40) – 20 = 100°2x – 30 = 2(40) – 30 = 50°

check,160° + 80° + 120° + 210° + 100° + 50° = 720°

720° = 720°

PolygonsPolygonsProblem SolvingProblem Solving

1) Find the sum of the angle measures in the figure to the left.

a) 180°b) 540°c) 720°d) 1260°

2) Find the angle measures in the polygon to the right.

a) m° = 150°b) m° = 144°c) m° = 120°d) m° = 90°

3) Give all the names that apply to the figure at the left.

a) quadrilateral, square, rectangle, rhombus, parallelogramb) quadrilateral, trapezoidc) quadrilateral, parallelogram, rectangle, squared) quadrilateral, parallelogram, trapezoid

4) Find the sum of the angle measures in a 20-gon. If the polygon is regular, find the measure of each angle.

a) 198°, 9.9°b) 720°, 72°c) 1800°, 90°d) 3240°, 162°

5) Find the value of the variable.

a) x° = 90°b) x° = 110°c) x° = 120°d) x° = 290°

6) Given the polygon at the left, what is the measure

of the interior angles?

A) 720B) 540C) 360D) 180