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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 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 • 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
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
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.
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.
12 units 3 units 3 units + 12 units = 15 units
Using the rectangle method, draw a rectangle around each of these triangles.
What observation can you make about these shapes and the shaded regions?
These triangles and ½ of the area of the rectangle. How will this information help us to find the area of the triangles?
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?
9 units 15 units Find the area of each rectangle by multiplying the base x height.
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.
4 ½ units 7 ½ units 4 ½ + 7 ½ = 12 units
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
To find the area, draw two right angles around the triangle to create a rectangle.
Find the area of the yellow and blue triangles. Use the rectangle method mentally.
7 ½ units 5 units Find the area of the yellow and blue triangles. Use the rectangle method mentally.
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.
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?
Use what you know to determine a strategy to find the area of this shape.
1 x 6 = 6 6 ÷ 2 = 3 = 3 units Determine the areas of the three shapes and add the parts.
1 x 6 = 6 6 ÷ 2 = 3 = 3 units Determine the areas of the three shapes and add the parts.
6 x 4 = 24 units Determine the areas of the three shapes and add the parts.
24 units = 3 units = 3 units 3 + 3 + 24 = 30 units
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
Math Journal page 316 Practice with a Partner
Math Sheet p. 126 Practice on Your Own