Geometry Problem. GeoGebra, HTML5 Animation for iPad and Nexus.

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Level: Mathematics Education, High School, Honors Geometry, College.

Click the figure below to see the dynamic geometry demonstration of problem 895.

## Wednesday, July 3, 2013

### Problem 895: Triangle, Incircle, Tangency Points, Parallel lines, Collinear Points

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Lemma 1: In trapezoid ABCD, let E is the intersection of diagonals AC and DB and F is the point of intersection of AD and BC then EF cut upper and lower bases at midpoints of the bases.

ReplyDeleteSee problem 714 for full demonstration.

http://img856.imageshack.us/img856/203/5d0.png

Let L and M are midpoints of GD and EH

1. Apply Lemma 1 in trapezoid GDHE , FH will cut GD and EH at midpoints of DG and EH

.=> F, L,K and M are collinear

2. From C draw a line parallel to AB . This line will cut EF and FD at E’ and D’ ( see sketch)

We have ∆(FAE) similar to ∆ (E’CE) ( case AA)

So ∆ (ECE’ )is isosceles and CE’=CE

Similarly ∆(FBD) similar to ∆(D’CD) => ∆( D’CD) is isosceles => CD=CD’

But CE=CD so CE’=CD’ => C is the midpoint of E’D’

3. Let line FLKM cut E’D’ at C’

With similar triangles we get C’E’/ME= FC’/FM=C’D’/MH

But ME=MH so C’E’=C’D’ => C’ is midpoint of E’D’ and C’ coincide to C

So F, K and C are collinear