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Geometry Difficulty 5.4 AIME, harder Prove it Taiwan

Let ABCDABCD be a convex quadrilateral. Points E,F,G,HE, F, G, H lie on segments AB,BC,CD,DAAB, BC, CD, DA respectively, such that lines AB,FH,CDAB, FH, CD, when extended, are concurrent, and lines BC,EG,ADBC, EG, AD are also concurrent. Let OO be the intersection point of EGEG and FHFH. Consider the following four quadrilaterals:
AHOE,BEOF,CFOGAHOE, BEOF, CFOG and DGOHDGOH.
Prove that: if three of these four quadrilaterals are tangential quadrilaterals, then the fourth must also be one.

Note: A quadrilateral is called a tangential quadrilateral if there exists a circle inside the quadrilateral such that the circle is tangent to all four sides.

Solution

Let XX be the intersection point of AB,FH,CDAB, FH, CD, and let YY be the intersection point of BC,EG,ADBC, EG, AD. Let L1,L2L_1, L_2 be the angle bisectors of AXO,OXC\angle AXO, \angle OXC respectively, and let K1,K2K_1, K_2 be the angle bisectors of AYO,OYC\angle AYO, \angle OYC respectively.
Clearly, the centers of the inscribed circles of the four quadrilaterals (if they exist) must lie at the intersection points of Li,KjL_i, K_j (i,j=1,2)(i, j = 1, 2). Let PijP_{ij} denote the intersection point of LiL_i and KjK_j. We set
α1=12AXO,α2=12OXC,β1=12AYO,β2=12OYC \alpha_1 = \frac{1}{2} \angle AXO, \quad \alpha_2 = \frac{1}{2} \angle OXC, \quad \beta_1 = \frac{1}{2} \angle AYO, \quad \beta_2 = \frac{1}{2} \angle OYC
The condition that the four quadrilaterals are tangential quadrilaterals is equivalent to the distance from PijP_{ij} to one pair of opposite sides being equal to the distance to the other pair of opposite sides, which can be written as
XPijsinαi=YPijsinβj XP_{ij} \sin \alpha_i = YP_{ij} \sin \beta_j
Now let aij=XPijsinαi,bij=YPijsinβja_{ij} = XP_{ij} \sin \alpha_i, b_{ij} = YP_{ij} \sin \beta_j, we want to prove that if aij=bija_{ij} = b_{ij} holds for three of (i,j)=(1,1),(1,2),(2,1),(2,2)(i, j) = (1, 1), (1, 2), (2, 1), (2, 2), then it also holds for the fourth.
Note that we have
XP11XP12YP12YP22XP22XP21YP21YP11=SXYP11SXYP12SXYP12SXYP22SXYP22SXYP21SXYP21SXYP11=1 \frac{XP_{11}}{XP_{12}} \cdot \frac{YP_{12}}{YP_{22}} \cdot \frac{XP_{22}}{XP_{21}} \cdot \frac{YP_{21}}{YP_{11}} = \frac{S_{XYP_{11}}}{S_{XYP_{12}}} \cdot \frac{S_{XYP_{12}}}{S_{XYP_{22}}} \cdot \frac{S_{XYP_{22}}}{S_{XYP_{21}}} \cdot \frac{S_{XYP_{21}}}{S_{XYP_{11}}} = 1

1=a11a12b12b22a22a21b21b11=a11b11a22b22b12a12b21a21 1 = \frac{a_{11}}{a_{12}} \cdot \frac{b_{12}}{b_{22}} \cdot \frac{a_{22}}{a_{21}} \cdot \frac{b_{21}}{b_{11}} = \frac{a_{11}}{b_{11}} \cdot \frac{a_{22}}{b_{22}} \cdot \frac{b_{12}}{a_{12}} \cdot \frac{b_{21}}{a_{21}}
Clearly, if three of the ratios on the right-hand side of the above equation are 11, then the fourth is also 11, which completes the proof!

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Source: MathNet, licensed CC-BY-4.0. Statement translated into English from zh; metadata (topic, difficulty) added by this project.