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Learning Math Home
Session 5, Part C: The Midline Theorem
 
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Session 5, Part C:
The Midline Theorem (55 minutes)

In This Part: The Midline Cut | Some Geometry Facts | Proving the Midline Theorem

In this part, we will begin to use segment notation. Optional: About Segment Notation.

Let's look again at how we solved Problem B3, in which we dissected a triangle to form a parallelogram. Note 4

Find the midpoints of two sides of a triangle. Cut along the segment connecting those two midpoints.

Rotate the top triangle 180° about one of the midpoints.

The two segments match because the cut was at the midpoint. The following are the conjectures that we will prove in the midline theorem:

 

Quadrilateral ABCD is a parallelogram because the opposite sides are the same length. AD and BC are the same length because they were made by cutting at a midpoint.

 

AB and CD are the same length because a midline cut makes a segment half as long as the base.

This activity requires the Flash plug-in, which you can download for free from Macromedia's Web site. If you prefer, you can view the non-interactive version of this activity, which doesn't require the Flash plug-in.


Next > Part C (Continued): Some Geometry Facts

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