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The Rectangle Method. Finding Area of Triangles and Parallelograms. Find something in the room shaped like: a hexagonal prism a cylinder a quadrangle a cube. A 30 Second Look-See. Read the top of page 315

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the rectangle method

The Rectangle Method

Finding Area of Triangles and Parallelograms

a 30 second look see

Find something in the room shaped like:

  • a hexagonal prism
  • a cylinder
  • a quadrangle
  • a cube
A 30 Second Look-See
math journal page 315

Read the top of page 315

  • Your job will be to think of ways to find the areas of the triangles – I am not interested in the answer.

I am interested in the methods you

and your partner discover.

Math Journal page 315
rectangle method

This is called the rectangle method because rectangles are used to surround the figure or parts of the figure. All of the areas that are calculated are either areas of rectangles or of triangular halves of rectangular regions.

Rectangle Method
slide6

Split this triangle up into two different triangles, both with right angles, count the units that cover each and find the sum of the two parts.

slide7

12 units

3 units

Split this triangle up into two different triangles, both with right angles, count the units that cover each and find the sum of the two parts.

slide11
These triangles and ½ of the area of the rectangle. How will this information help us to find the area of the triangles?
slide12

12 units

3 units

These triangles and ½ of the area of the rectangle. How will this information help us to find the area of the triangles?
slide16

9 ÷ 2 =

4 ½ units

15 ÷ 2 =

7 ½ units

To find the area of the triangles divide the area of the rectangles by 2 or find the value of ½ the rectangle.
review your work

Return to page 315.

  • Find the area of triangle #1 by splitting the triangle into two right triangles. Use the add the parts method to find the area of the complete triangle.
Review your work
slide23
To find the area of the red triangle, find the area of the rectangle and subtract the 12 ½ units of the blue and yellow triangles.

The difference will be the area of the red triangle.

slide24

15 units – 12 ½ units = 2 ½ units.

If the yellow triangle is 7 ½ units , and the blue triangle is 5 units, does it make sense that the red triangle is 2 ½ units?
review your work2

Return to page 315.

  • Find the area of triangle #3 by splitting the parallelogram into two right triangles and a rectangle. Use the add the parts method to find the area of the complete parallelogram.
Review your work
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