How to Compute Perimeter and Circumference

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Mathematics, Measurement (includes Time), Operations (+, -, x, /, etc.)

Grade 5- 8

Objective

Students learn how to compute perimeter and circumference.

Directions

Discuss with students the following facts. Go over the sample problems as a class.
Facts to Know

  • Perimeter is the distance around all the edges of an object.
  • The distance around a property such as a house, lot, or a schoolyard is the perimeter of the property.
  • The following words usually indicate perimeter: border, walls, fence, edges, sides, or distance around.
    Perimeters of Rectangles
    The perimeter of a rectangle can be computed by any of the following ways:
  • adding up all four sides of the rectangle
  • adding the length and width and multiplying by 2
  • using the formula: P = (l + w) x 2 or P = 2(l + w)
    (Note: P = perimeter, l = length, and w = width)
    Compute the perimeter of this rectangle. The length is 3.5 centimeters and the width is 2.4 centimeters.
    Add the 3.5 plus 2.4, which equals 5.9 centimeters.
    Multiply by 2 to get the other two sides of the rectangle.
    The total perimeter is 11.8 centimeters.
    Perimeters of Other Polygons


    Add the sides of each polygon to compute the perimeter.
    6.3 cm + 6.3 cm + 4.5 cm = 17.1 cm 7.3 cm x 5 = 36.5 cm
    Circumference of a Circle
    The formula for determining the circumference of a circle is 2πr or πd. (To solve, multiply 2 times r (the radius) times π (equals to approximately 3.14) or multiply d (the diameter) times 3.14.
    Multiply 5 times 2 times 3.14 or
    5 x 2 x 3.14 = 31.4 m, so C = 31.4 meters

    • Resources

    • activity sheets (3 per student)
    • pencils

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