It's a challenge, can you do it-MCQ

Geometry Level 4

A D AD , B E BE and C F CF are concurrent lines drawn from the vertices of A B C \triangle ABC to points D D , E E and F F on the opposite sides. If A D AD is the altitude of A B C \triangle ABC , then find the ratio between the F D A \angle FDA and E D A \angle EDA .

Depends on configuration of figure 2 √2 1 3 2

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1 solution

Mark Hennings
Nov 16, 2017

This result is known as Blanchet's Theorem.

Draw a line through A A parallel to B C BC . Then the triangles A M E AME and C D E CDE are similar, as are the triangles A N F ANF and B D F BDF . Thus we deduce that A E E C = A M C D A F F B = A N B D \frac{AE}{EC} \; = \; \frac{AM}{CD} \hspace{2cm} \frac{AF}{FB} \; = \; \frac{AN}{BD} Ceva's Theorem for the triangle A B C ABC tells us that A F F B × B D D C × C E A E = 1 \frac{AF}{FB} \times \frac{BD}{DC} \times \frac{CE}{AE} \; = \; 1 and hence we deduce that A M = A N AM = AN . Thus the right-angled triangles A M D AMD and A N D AND are congruent, and hence F D A = E D A \angle FDA \,=\, \angle EDA , so the desired ratio is 1 \boxed{1} .

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