🔢 MathGrade 3Lesson 26
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Classify Triangles and Quadrilaterals

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What you will learn

  • 1Classify triangles by side lengths and angles
  • 2Identify and name quadrilaterals by their properties
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Mini Lesson

Triangles and quadrilaterals can be sorted into different types based on their side lengths and angles. Knowing the properties of each type helps us identify and name them correctly.

Types of Triangles

  • Equilateral triangle: all 3 sides equal, all 3 angles equal (60 degrees each).
  • Isosceles triangle: exactly 2 sides equal, 2 base angles equal.
  • Scalene triangle: no sides equal, no angles equal.
  • Right triangle: has exactly one 90-degree angle.

Types of Quadrilaterals

  • Square: 4 equal sides, 4 right angles.
  • Rectangle: opposite sides equal and parallel, 4 right angles.
  • Rhombus: 4 equal sides, opposite angles equal, no right angles.
  • Trapezoid: exactly 1 pair of parallel sides.
  • Parallelogram: 2 pairs of parallel sides, opposite sides equal.

Words to know

Triangle

A polygon with 3 sides

e.g. A triangle has 3 sides and 3 angles.

Quadrilateral

A polygon with 4 sides

e.g. A square is a quadrilateral.

Equilateral

Having all sides the same length

e.g. An equilateral triangle has 3 equal sides.

Isosceles

A triangle with two equal sides

e.g. An isosceles triangle has two sides the same.

Rectangle

A quadrilateral with 4 right angles

e.g. A rectangle has 2 long sides and 2 short sides.

Rhombus

A quadrilateral with 4 equal sides

e.g. A diamond shape is a rhombus.

Three triangle types side by sideThree triangles: equilateral with 3 tick marks on all sides, isosceles with 2 tick marks, and right with a square corner marker.Types of TrianglesEquilateral3 equal sidesIsosceles2 equal sidesRight1 right angle
Four quadrilateral types in a two by two gridFour quadrilaterals: square, rectangle, rhombus, and trapezoid, each labeled with its key property.Types of QuadrilateralsSquare4 equal sides, 4 right anglesRectangleOpposite sides equalRhombusTrapezoid
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Tick marks on sides show equal lengths. A square corner marker shows a right angle.

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← Lesson 25: Identify Polygons and AnglesLesson 27: Recognize Symmetry and Patterns →