Maths Olympiad Prep

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Geometry Difficulty 6.5 National olympiad Prove it

EE is a point on the diagonal BDBD of the square ABCDABCD. Show that the points A,EA, E and the circumcenters of ABEABE and ADEADE form a square.

Solution

1. Define the points and circumcenters:
Let O1 O_1 and O2 O_2 be the circumcenters of the triangles ABE ABE and ADE ADE , respectively. We need to show that the points A,E,O1, A, E, O_1, and O2 O_2 form a square.

2. Circumradius equality:
Let R1 R_1 and R2 R_2 be the lengths of the circumradii of the triangles ABE ABE and ADE ADE , respectively. Using the Law of Sines in triangles ABE ABE and ADE ADE , we have:
2R1=ABsinAEBand2R2=ADsinAED 2R_1 = \frac{AB}{\sin \angle AEB} \quad \text{and} \quad 2R_2 = \frac{AD}{\sin \angle AED}
Since AB=AD AB = AD (both are sides of the square ABCD ABCD ) and AEB=AED \angle AEB = \angle AED (as E E lies on the diagonal BD BD ), it follows that:
2R1=2R2    R1=R2 2R_1 = 2R_2 \implies R_1 = R_2
Therefore, the circumradii are equal, and we have:
AO1=O1E=R1=R2=EO2=O2A AO_1 = O_1E = R_1 = R_2 = EO_2 = O_2A

3. Angle calculation:
Next, we need to show that AO1E=90 \angle AO_1E = 90^\circ . The angle subtended by the arc AE AE in the circumcircle of ABE \triangle ABE is twice the angle at B B :
AO1E=2ABE \angle AO_1E = 2 \cdot \angle ABE
Since ABE=45 \angle ABE = 45^\circ (as ABE \triangle ABE is a right triangle with ABE \angle ABE being half of 90 90^\circ ), we have:
AO1E=245=90 \angle AO_1E = 2 \cdot 45^\circ = 90^\circ

4. Conclusion:
From the equal circumradii and the right angle, we conclude that AO1EO2 AO_1EO_2 is a square. The sides are equal, and the angles are right angles.

\blacksquare

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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.