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Geometry Difficulty 6.5 National olympiad Prove it

Let ABCDA B C D be a trapezoid with ABA B parallel to CD,AB>CDC D,|A B|>|C D|, and equal edges AD=BC|A D|=|B C|. Let II be the center of the circle tangent to lines AB,ACA B, A C and BDB D, where AA and II are on opposite sides of BDB D. Let JJ be the center of the circle tangent to lines CD,ACC D, A C and BDB D, where DD and JJ are on opposite sides of ACA C. Prove that IC=JB|I C|=|J B|.

Solutions — 2

Solution 1

Let {P}=ACBD\{P\}=A C \cap B D and let APB=1802a\angle A P B=180-2 a. Since ABCDA B C D is an isosceles trapezoid, APBA P B is an isosceles triangle. Therefore PBA=a\angle P B A=a, which implies that PBI=90a/2\angle P B I=90^{\circ}-a / 2 since II lies on the external bisector of PBA\angle P B A. Since II lies on the bisector of CPB\angle C P B, it follows that BPI=a\angle B P I=a and hence that IPBI P B is isosceles with IP=PB|I P|=|P B|. Similarly JPCJ P C is isosceles with JP=PC|J P|=|P C|. So, in the triangles CPIC P I and BPJB P J we have PIPBP I \equiv P B and PJCPP J \equiv C P. Since II and JJ both lie on the internal bisector of BPC\angle B P C, it follows that triangles CPIC P I and BPJB P J are congruent. Therefore IC=JB|I C|=|J B|.

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Solution 2

1. Identify the properties of the trapezoid:
- Given ABCDABCD is a trapezoid with ABCDAB \parallel CD and AB>CD|AB| > |CD|.
- The edges AD=BC|AD| = |BC| are equal, indicating that ABCDABCD is an isosceles trapezoid.

2. Intersection of diagonals:
- Let ACBD=OAC \cap BD = O. In an isosceles trapezoid, the diagonals intersect at a point OO such that OAC=OBD\angle OAC = \angle OBD and OAD=OBC\angle OAD = \angle OBC.

3. Angles at the intersection point:
- Since ABCDAB \parallel CD, the angles OAC=OBD=θ\angle OAC = \angle OBD = \theta and OAD=OBC=θ\angle OAD = \angle OBC = \theta.

4. Centers of the circles:
- Let II be the center of the circle tangent to lines ABAB, ACAC, and BDBD.
- Let JJ be the center of the circle tangent to lines CDCD, ACAC, and BDBD.

5. Angles involving the centers:
- Since II is the center of the circle tangent to ABAB, ACAC, and BDBD, the angle OAI=θ2\angle OAI = \frac{\theta}{2} because ACAC is tangent to the circle with center II.
- Similarly, AOI=180θ\angle AOI = 180^\circ - \theta implies OIA=θ2\angle OIA = \frac{\theta}{2}.

6. Equal distances from the intersection point:
- From the above, we have OI=OBOI = OB because OAI=OIA\angle OAI = \angle OIA and OO is the midpoint of ACAC and BDBD in terms of the tangency points.

7. **Similar argument for JJ:**
- By a similar argument, OJ=OCOJ = OC because OAJ=OJA\angle OAJ = \angle OJA and OO is the midpoint of ACAC and BDBD in terms of the tangency points.

8. Congruent triangles:
- We also have BOJ=IOC\angle BOJ = \angle IOC because both are formed by the intersection of the diagonals and the tangency points.
- Combining OI=OBOI = OB, OJ=OCOJ = OC, and BOJ=IOC\angle BOJ = \angle IOC, we get BOJIOC\triangle BOJ \cong \triangle IOC by the Angle-Side-Angle (ASA) criterion.

9. Conclusion:
- Since BOJIOC\triangle BOJ \cong \triangle IOC, it follows that IC=JB|IC| = |JB|.

\blacksquare

The final answer is IC=JB|IC| = |JB|

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Source: NuminaMath-1.5, licensed Apache-2.0. Statement and solution reproduced as published; topic and difficulty added by this site.