Section 9.3 ~Classifying Polygons

Pages 470 - 474

A polygon is a closed plane figure with at least three sides. The sides meet only at their endpoints.

A triangle is a polygon with three sides. You can classify triangles by angle measures. You can also classify triangles by side lengths. Tick marks are used to indicate congruent sides of a figure.

Classifying a Triangle

Classify the triangle by its sides and angles.

The triangle has two congruent sides and one right angle. The triangle is an isosceles right triangle.

Judging by appearance, classify the triangle by its sides and angles.                             a. scalene right triangle      b. isosceles right triangle      c. isosceles obtuse triangle      d. scalene acute triangle

Judging by appearance, classify the triangle by its sides and angles.                        a. scalene right triangle      b. isosceles right triangle      c. isosceles obtuse triangle      d. scalene acute triangle Judging by appearance, classify the triangle by its sides and angles.                                    a. scalene obtuse triangle      b. isosceles acute triangle      c. isosceles right triangle      d. scalene right triangle

You can also classify quadrilaterals by their sides and angles.

                               All parallelograms have opposite sides parallel. Parallelograms include rectangles, rhombuses, and squares.

Classifying Quadrilaterals

  Name the types of quadrilaterals that have both pairs of opposite sides parallel.

In later math courses, you will prove that a parallelogram has opposite sides congruent and opposite angles congruent.

Polygons are named using their vertices. Start at one vertex and list them in consecutive order.

A regular polygon has all sides congruent and all angles congruent. Some regular polygons are shown below.

                  Real-World      Problem Solving

A contractor is framing a regular octagonal gazebo. Write a formula for the perimeter of the gazebo in terms of the length of a side. Evaluate the formula for a side length of 7 ft.

To write a formula, let x = the length of each side.

The perimeter of the regular octagon is x + x + x + x + x + x + x + x.

Therefore the formula for the perimeter is P = 8x.

For a side length of 7 ft, the perimeter is 56 ft.

Write a formula to find the perimeter of a regular hexagon.      a. P = 6 + x      b. P = 5x      c. P = 5 + x      d. P = 6x

Use the formula to find the perimeter if one side is 16 cm.      a. 80 cm      b. 21 cm      c. 96 cm      d. 22 cm

Check Understanding

Homeworkpages 473 - 474

#s 1-25, 32-35– draw diagrams AND– write problems and answers, EXCEPT

for word problems and multiple-choice problems

– SHOW ALL WORK!– failure to write problems and/or show

work will result in either receiving partial or no credit