Given triangle ABC with side lengths a, b, and c. Let D, E, and F be the points of tangency of the incircle, as shown.

(a) Prove that triangle DEF is acute, that is, that the triangle determined by the points of tangency of the incircle is always acute.

(b) Find the area of triangle DEF in terms of a, b, and c.(c) Show that AD, BF, and CE are concurrent. Explore this point of concurrency as the shape of triangle ABC is varied.

Hints/Solution:

Clickherefor a GSP sketch of the incircle.The triangle DEF is the

Pedal Trianglewhen the Pedal Point is the incenter I.

Strategy: Consider the area of triangle DEF as the sum of the areas of trianglesEIF,DIE, andFID.

Comment:Compare with the

Excircle Problems

Reference:For an extension and partial solution, see problem 4153 inSchool Science and, October 1987 and October 1988 issues.Mathematics

In Problem 4153, A(I) is the area of the triangle from the tangent points of the incircle, and A(E1), A(E2), and A(E3) are the areas of the three triangles from the tangent points of the three excircles.

Click

hereto see a picture. Clickherefor a Geometer's Sketchpad file.

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