In acute △ A B C , let D , E , F be the feet of perpendiculars from A , B , C to B C , C A , A B respectively. Line E F meets the circumcircle of △ A B C at two distinct points P and Q . D F intersects B P and B Q at points R , S respectively. Also, line D E intersects C Q and C P at points T , U respectively. Find A P + A Q + A U A R + A S + A T .
This problem is not original. I had fun working on it, so I thought it would be worth sharing.
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Nice question.
As a side note, the fact that A P = A Q is immediate, because E F is parallel to the tangent at A (slight angle chasing, show that ∠ A F E = ∠ A C B ).
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I have shown that ∠ A F E = ∠ A C B in my solution. Could you please explain how A P = A Q follows from the fact that E F is parallel to the tangent at A ? Am I missing something obvious here?
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It's an almost obvious statement, which follows from symmetry of the circle. Let ℓ 1 be any line that intersects a circle at points K and L . Let ℓ 2 be a line that is parallel to ℓ 1 , and intersects the same circle at points X and Y . Then by symmetry of the circle, K X = L Y .
A more rigorous argument would be that the perpendicular bisectors of K L and X Y are parallel, and pass through the same point (center of the circle), hence they are the same. This shows that ∠ K O X = 2 1 ( ∠ K O L − ∠ X O Y ) = ∠ L O Y (I'm avoiding using directed angles, but you should easily see the equality of angles ). Hence K X = L Y .
Of course, we can have ℓ 1 be tangential to the circle, which gives us that A P = A Q as claimed.
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Consider the simplified diagram above. Let H be the orthocenter of △ A B C . Since H D ⊥ B C and H F ⊥ A B , quadrilateral B F H D is cyclic with diameter B H . Note that ∠ B F D = = = = = ∠ B H D ∠ A H E 9 0 ∘ − ∠ H A E 9 0 ∘ − ( 9 0 ∘ − ∠ B C A ) ∠ B C A . Similarly, ∠ A F E = ∠ B C A . Since quadrilateral A B C P is cyclic, ∠ B P A = ∠ B C A . Let ∠ P B C = ∠ P A C = θ . Then, ∠ A B R = ∠ A B C − θ , and ∠ B R F = 1 8 0 ∘ − ∠ B C A − ( ∠ A B C − θ ) = ∠ B A C + θ = ∠ F A P . Now note that ∠ F R P = 1 8 0 ∘ − ∠ B R F = 1 8 0 ∘ − ∠ F A P , so quadrilateral A F R P is cyclic. We then have ∠ A R P = ∠ A F P = ∠ B C A = ∠ A P B , so △ A R P is isoceles with A P = A R . In a similar manner, we can prove that A R = A S = A T = A P = A Q = A U . Our desired answer is A P + A Q + A U A R + A S + A T = 1 .
This problem is adapted from IMOSL 2010 G1 .