🔢 MathGrade 3Lesson 25
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Identify Polygons and Angles

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What you will learn

  • 1Classify polygons as triangles, quadrilaterals, or other polygons
  • 2Identify right, acute, and obtuse angles
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Mini Lesson

A polygon is a flat, closed shape made of straight sides. Polygons are named by how many sides they have. Triangles have 3 sides. Quadrilaterals have 4 sides. Pentagons have 5, hexagons have 6, and octagons have 8 sides.

Classifying Polygons

  • Triangle: 3 sides, 3 angles, and 3 corners (each corner is a vertex). Examples: equilateral, isosceles, right triangle.
  • Quadrilateral: 4 sides and 4 angles. Examples: square, rectangle, trapezoid.
  • Pentagon: 5 sides. Hexagon: 6 sides. Octagon: 8 sides.
  • A regular polygon has all sides and all angles equal; if they are not all equal, it is an irregular polygon.

Types of Angles

  • Right angle: exactly 90 degrees. Looks like a square corner. Marked with a small square.
  • Acute angle: less than 90 degrees. Sharp and small.
  • Obtuse angle: greater than 90 degrees but less than 180 degrees. Wide and open.

Words to know

Polygon

A closed shape with straight sides

e.g. Triangles and squares are polygons.

Angle

The corner where two sides of a shape meet

e.g. A square has 4 angles.

Right angle

A square corner — exactly 90 degrees

e.g. A rectangle has 4 right angles.

Acute angle

An angle smaller than a right angle

e.g. A triangle can have acute angles.

Obtuse angle

An angle bigger than a right angle but less than 180 degrees

e.g. An obtuse angle is wider than 90 degrees.

Vertex

A corner of a shape where two sides meet

e.g. A triangle has 3 vertices.

Six polygon shapes in a gridSix polygon shapes arranged in two rows: triangle, square, pentagon in row 1, and hexagon, trapezoid, octagon in row 2.PolygonsTriangleSquarePentagonHexagonTrapezoidOctagonPolygons are named by number of sides
Three angle types: right, acute, obtuseThree angle diagrams: a right angle with a square corner marker, an acute angle smaller than 90 degrees, and an obtuse angle greater than 90 degrees.Types of AnglesRight= 90°Acute< 90°Obtuse> 90°Angles are measured in degrees
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A right angle is like the corner of a square or the corner of a book. Acute angles are smaller; obtuse angles are bigger.

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← Lesson 24: Measure Area with Unit SquaresLesson 26: Classify Triangles and Quadrilaterals →