Lesson Plan # 4 - Perimeter and composite rectangular shapes

  1. (5 min) Mental Math
    1. Add the following numbers: 8,12,10, and 5 [35]
    2. Start with 48, divide by 6, then add 10 [18]
    3. What is 2.5 + 2.5? [5]
    4. Start with the number of inches in a foot, then add the number of feet in a yard. [15]
    5. Start with 16, double it, then add 8. [40]

  2. (5 min) Review of selected problems from Homework # 3 (<= 3 problems)
  3. (5 min) Introduce rectangle perimeter. Use the "dot" method:

    Example #1:

      Perimeter: "Put a dot in the upper
                  left corner then go 
                  clockwise, adding sides
                  as you go".
      10  +  3  +  10  +  3  =  26 feet
    

    Example #2:                         Example #3:
    

    Perimeter = 12 + 6 + 12 + 6   Perimeter = 8.5 + 2.5 + 8.5 + 2.5
              = 36 cm                       = 22ft.
    

    Example #4:

    Perimeter = 6.5 + 5.5 + 6.5 + 5.5 = 24 inches
  4. (15 min) In-class exercise - page 1

    Note: Students have a tendency to only count the labelled sides. The "dot" method addresses this problem by getting the student to follow around the whole figure, adding one side at a time. This becomes more important when composite shapes are involved.
  5. (10 min) Introduce composite rectangular shapes.

    Technique: cut into 2 or more rectangles.

    Example 1:


    
    Area #1 = 2 x 2 =  4 
    Area #2 = 8 x 2 = 16
                     ---
               Total: 20 sq. in.
    


    Example 2:


    Area #1 = 8 x 4 = 32 
    Area #2 = 6 x 5 = 30
                     ---
               Total: 62 sq. in.
    


    It is important to get students to draw the line(s) separating the rectangles from each other. It also helps to have them write the little "number in the circle" indicating the identity of each rectangle, so they can be sure they have added it's area to t he total.
    Have students fill in missing dimensions, compute areas and suggest different ways to cut figures up.
  6. (Remainder of class) In-class exercise - page 2
  7. Hand out homework to students as they successfully complete the in-class exercise.