1. Two-Dimensional Figures
Geometry
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Warm Up
Classify the angle as acute; right angle, obtuse or straight. 2) 2) 1) 1)
Acute
Right angle 3) 3)
Obtuse 4) 4)
Straight
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Polygon
Flat figures such as squares, rectangles are two-dimensional figures.
A polygon is a simple, closed, two-dimensional figure formed by three or more sides.
The number of sides determine a polygon’s name.
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Polygon
Triangle
(3 sides)
Pentagon
(5 sides)
Quadrilateral
(4 sides)
Octagon
(8 sides)
Hexagon
(6 sides)
Heptagon
(7 sides)
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Polygon
Tri: three
Quad: four
Penta: five
Hexa: six
Hepta: seven
Octa: eight
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Polygon
Lateral: comes from the Latin word meaning side
-gon: comes from the Greek word for angle
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Regular Polygon
When all sides of a polygon are congruent and all of the angles are congruent, the polygon is a regular polygon
The polygon has 5 sides. So, it is a pentagon. Since the sides and angles are congruent, it is a regular polygon.
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Your Turn!
Identify the polygon. Then tell if it is a regular polygon.
The polygon has 7 sides. So, it is a heptagon. Since the sides and angles are not congruent, it is not a regular polygon.
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Triangles
Step 1: Draw a triangle similar to the one shown below. Then tear off each corner.
Step 2: Rearrange the torn pieces as shown. 1 1 2 3 2 3
When you arrange the angles, a line is formed.
By extending the line, you find 180° on the protractor.
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Triangles
Step 3: Repeat Steps 1 and 2 with a different triangle.
Therefore, the sum of the measures of the angles of a triangle.
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Triangles
Triangles can be classified according to their angles.
Acute
Right
Obtuse
All angles are acute
One angle is right angle
One angle is obtuse
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Your Turn!
Find the missing angle measure for each triangle. Then classify each triangle as acute, right, or obtuse.
30° x°
45°
60°
63° x°
95° x°
20°
40°
Obtuse
57°
Acute
130°
Obtuse
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13. Special names of triangles
Figure
Characteristic
Scalene Triangle
No sides congruent
Isosceles triangle
At least two sides are congruent
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14. Special names of triangles
Figure
Characteristic
Equilateral triangle
All sides are congruent
All angles are congruent
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Quadrilaterals
Quadrilaterals can be classified by their sides and angles.
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16. Special names of quadrilaterals
Figure
Characteristic
Rectangle
Opposite sides congruent
All angles are at right angles
Opposite sides are parallel
Square
All sides are congruent
All angles are right angles
Opposite sides are parallel
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17. Special names of quadrilaterals
Figure
Characteristic
Parallelogram
Opposite sides congruent
Opposite sides parallel
Opposite angles congruent
Rhombus
All sides congruent
Opposite sides parallel
Opposite angles congruent
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Your Turn!
How are rectangles and squares alike? How do they differ?
Sample answer: similarities- four sides, closed figures, opposite sides equal, opposite sides parallel; differences-squares have four congruent sides
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Assessment
Draw an example of each polygon listed. Mark any congruent sides, congruent angles, and right angles.
Hexagon
Regular octagon
Parallelogram
Triangle
Equilateral Triangle
Rectangle
Check student’s responses
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Assessment
Tell whether each statement is sometimes, always, or never true.
Parallelograms are squares sometimes
A rhombus is a square sometimes
A rectangle is a parallelogram always
A square is a rhombus always
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Review
A polygon is a simple, closed, two-dimensional figure with three or more sides.
When all the sides and angles of a polygon are congruent, then the polygon is congruent.
Scalene, Isosceles and Equilateral are three types of triangles
Rectangle, square, parallelogram, and rhombus are types of quadrilaterals
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Thank You!
Remember to do the practice worksheets!!!
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