Gr 5 MG 1-1 tri area

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Connections In Areas: Rectangles & Triangles By Renee Hill Stephanie Lessman Linda Stein TTTC Math & Technology Summer 2001

Today’s Goal: Determine the formula for the area of a triangle. : 

Today’s Goal: Determine the formula for the area of a triangle. California Standard – Grade Five Measurement and Geometry 1.1 (partial) Derive and use the formula for the area of a triangle by comparing it with the formula for the area of a rectangle.

What are the properties of a rectangle?

Properties of a Rectangle : 

Properties of a Rectangle There are four right angles.

Properties of a Rectangle : 

Properties of a Rectangle There are two sets of parallel sides opposite one another.

Area of Rectangle : 

Area of Rectangle Identify the area of the rectangle. This is the area of the rectangle.

Calculating the Area : 

Calculating the Area Determine the area of this rectangle if the length is 8 units and the width is 3 units. 3 units 8 units The area of this rectangle is 24 square units.

Writing the Equation and the Formula : 

Writing the Equation and the Formula 3 units 8 units The area of this rectangle is 24 square units. width length times equals area 8 units x 3 units = 24 square units Equation: Formula:

The Rectangle-Triangle Connection : 

The Rectangle-Triangle Connection How can you divide this rectangle into two equal triangles?

The Solution : 

The Solution There are two solutions.

The Triangle-Rectangle Connection : 

The Triangle-Rectangle Connection What is the area of the triangle A and triangle B combined? Triangle A Triangle B

The Solution : 

The Solution The combination of Triangle A and Triangle B has an area equal to the rectangle. Triangle A Triangle B

The Triangle Disconnection : 

The Triangle Disconnection Is it possible to calculate the area of one of these triangles? Yes!

Half-Size It : 

Half-Size It 3 units 8 units Note that Triangle A is half of the area of the rectangle.

Where We Started : 

Where We Started 3 units 8 units Remember that the area of the rectangle was 24 square units?

Writing the Equation : 

Writing the Equation 3 units 8 units The rectangle area equation was: The triangle area equation is half the rectangle: ( ) 12

Equation vs. Formula : 

Equation vs. Formula The triangle area equation is: ( ) 12 The triangle area formula uses the labels in place of the numbers.

The Equation – Formula Connection : 

The Equation – Formula Connection 8 units x 3 units = square units The triangle area equation is: ( ) 12 length x width = area The triangle area formula is: ( )

Use What You Know : 

Use What You Know Use your knowledge of rectangles and triangles to determine the area of more complex figures. l x w (l x w)

What’s Next? : 

What’s Next? In our next lessons, we will explore these questions: and . . . What about circles? What is the formula for the area of a parallelogram? Does this work for all types of triangles?