Figure 1 shows Δ *ABC* with *D* and *E* as midpoints of sides *AC *and *AB *respectively. If you look at this triangle as though it were a trapezoid with one base of *BC *and the other base so small that its length is virtually zero, you could apply the “median” theorem of trapezoids, *Theorem 55.*

**Figure 1 **The segment joining the midpoints of two sides of a triangle.

*Theorem 56 (Midpoint Theorem):* The segment joining the midpoints of two sides of a triangle is parallel to the third side and half as long as the third side.

In Figure 1, by *Theorem 56,*

**Example 1:** In Figure 2, find *HJ.*

**Figure 2 **Compute the length of the broken line segment joining the midpoints of two sides of the triangle.

Because *H* and *J* are midpoints of two sides of a triangle:

True or False: All triangles are convex

What is the degree measure of the interior angle determined by two adjacent sides of a regular decagon?

144°