Find The Ratio

Geometry Level 4

In acute A B C , \triangle ABC, let D , E , F D, E, F be the feet of perpendiculars from A , B , C A, B, C to B C , C A , A B BC, CA, AB respectively. Line E F EF meets the circumcircle of A B C \triangle ABC at two distinct points P P and Q . Q. D F DF intersects B P BP and B Q BQ at points R , S R, S respectively. Also, line D E DE intersects C Q CQ and C P CP at points T , U T, U respectively. Find A R + A S + A T A P + A Q + A U . \dfrac{AR+AS+AT}{AP+AQ+AU}.

This problem is not original. I had fun working on it, so I thought it would be worth sharing.


The answer is 1.00.

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1 solution

Image link: http://s29.postimg.org/6ekigv03r/Untitled.png Image link: http://s29.postimg.org/6ekigv03r/Untitled.png

Consider the simplified diagram above. Let H H be the orthocenter of A B C . \triangle ABC. Since H D B C HD \perp BC and H F A B , HF \perp AB, quadrilateral B F H D BFHD is cyclic with diameter B H . BH. 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 . \begin{array}{lcl} \angle BFD & = & \angle BHD \\ & = & \angle AHE \\ & = & 90^{\circ} - \angle HAE \\ & = & 90^{\circ} - (90^{\circ} - \angle BCA) \\ & = & \angle BCA. \end{array} Similarly, A F E = B C A . \angle AFE = \angle BCA. Since quadrilateral A B C P ABCP is cyclic, B P A = B C A . \angle BPA = \angle BCA. Let P B C = P A C = θ . \angle PBC = \angle PAC= \theta. Then, A B R = A B C θ , \angle ABR = \angle ABC - \theta, and B R F = 18 0 B C A ( A B C θ ) = B A C + θ = F A P . \angle BRF = 180^{\circ} - \angle BCA - (\angle ABC - \theta ) = \angle BAC + \theta = \angle FAP. Now note that F R P = 18 0 B R F = 18 0 F A P , \angle FRP = 180^{\circ} - \angle BRF = 180^{\circ} - \angle FAP, so quadrilateral A F R P AFRP is cyclic. We then have A R P = A F P = B C A = A P B , \angle ARP = \angle AFP = \angle BCA = \angle APB, so A R P \triangle ARP is isoceles with A P = A R . AP= AR. In a similar manner, we can prove that A R = A S = A T = A P = A Q = A U . AR= AS= AT = AP= AQ= AU. Our desired answer is A R + A S + A T A P + A Q + A U = 1 . \dfrac{AR+AS+AT}{AP+AQ+AU} = \boxed{1}.


This problem is adapted from IMOSL 2010 G1 .

Nice question.

As a side note, the fact that A P = A Q AP = AQ is immediate, because E F EF is parallel to the tangent at A A (slight angle chasing, show that A F E = A C B \angle AFE = \angle ACB ).

Calvin Lin Staff - 7 years, 2 months ago

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I have shown that A F E = A C B \angle AFE = \angle ACB in my solution. Could you please explain how A P = A Q AP= AQ follows from the fact that E F EF is parallel to the tangent at A ? A? Am I missing something obvious here?

Sreejato Bhattacharya - 7 years, 2 months ago

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It's an almost obvious statement, which follows from symmetry of the circle. Let 1 \ell_1 be any line that intersects a circle at points K K and L L . Let 2 \ell_2 be a line that is parallel to 1 \ell_1 , and intersects the same circle at points X X and Y Y . Then by symmetry of the circle, K X = L Y KX = LY .

A more rigorous argument would be that the perpendicular bisectors of K L KL and X Y XY are parallel, and pass through the same point (center of the circle), hence they are the same. This shows that K O X = 1 2 ( K O L X O Y ) = L O Y \angle KOX = \frac{1}{2} ( \angle KOL - \angle XOY) = \angle LOY (I'm avoiding using directed angles, but you should easily see the equality of angles ). Hence K X = L Y KX = LY .

Of course, we can have 1 \ell_1 be tangential to the circle, which gives us that A P = A Q AP = AQ as claimed.

Calvin Lin Staff - 7 years, 2 months ago

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