, and are concurrent lines drawn from the vertices of to points , and on the opposite sides. If is the altitude of , then find the ratio between the and .
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This result is known as Blanchet's Theorem.
Draw a line through A parallel to B C . Then the triangles A M E and C D E are similar, as are the triangles A N F and B D F . Thus we deduce that E C A E = C D A M F B A F = B D A N Ceva's Theorem for the triangle A B C tells us that F B A F × D C B D × A E C E = 1 and hence we deduce that A M = A N . Thus the right-angled triangles A M D and A N D are congruent, and hence ∠ F D A = ∠ E D A , so the desired ratio is 1 .