Reflection: Coherence Finding the Area of a Trapezoid - Section 2: Finding the Area of a Trapezoid Using the Triangle Formula


As students worked on finding the area of trapezoids  by decomposing the figure into triangles, they were confused by where to place the diagonal and what line indicates the height.  This is how I helped them.

First, I drew the height both inside and outside of the shape.  This way they could see the height of both triangles. 

Next, I said they should start with the vertice that had the right angle and draw the diagonal to the other vertice.

Finally, I asked them to tell me  ways to identify the height:  The base and height form a right angle, it is typically marked with a dotted line, and it is a straight up and down piece.

 Additionally, I always make the students write down the formula for the shape they are using.  This way, they can substitute in the correct numbers for the expression and it helps keep them organized on paper.  This is a great strategy to use for all kids, but especially for the special education students. 


  Common misconception
  Coherence: Common misconception
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Finding the Area of a Trapezoid

Unit 8: Geometry
Lesson 4 of 17

Objective: SWBAT find the area of a trapezoid by decomposing it into triangles and using the formula

Big Idea: Finding area and using formulas is a skill needed for Algebra and Geometry

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