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Geometry Difficulty 6.7 National olympiad Prove it Ukraine

On the side BCBC of acute triangle ABCABC one chooses an arbitrary point DD. Let OO be the circumcenter of ABC\triangle ABC, ZZ the point on this circle which is diametrically opposite to AA. Let XX, YY be points on segments BOBO, COCO such that:
BXD+ABC=180=CYD+ACB. \angle BXD + \angle ABC = 180^{\circ} = \angle CYD + \angle ACB.

Prove that the measure of angle XZY\angle XZY doesn't depend on point DD.
(Khilko Danylo)

Solution

Here is the solution for the location of points showed in the picture. For other cases, the location of the solution will be similar. Prove that for any selected point DD quadrilateral XOYZXOYZ is inscribed. Then XZY=1802A\angle XZY = 180^{\circ} - 2\angle A, so doesn't depend on DD.

From the statement of the problem we have that circles BXD\triangle BXD and DYC\triangle DYC are tangent to lines ABAB and ACAC. Denote these circles as w1w_1 and w2w_2, and their centers are O1O_1 and O2O_2. Let KK be the second point of intersection of these circles other than DD (see figure below).

Figure 1

Then
BKC=BKD+DKC=B+C=180A, \angle BKC = \angle BKD + \angle DKC = \angle B + \angle C = 180^{\circ} - \angle A,
so KK is on the circumcircle ABC\triangle ABC. Also we have
XKY=XKD+DKY=XBD+DCY=180XOY, \angle XKY = \angle XKD + \angle DKY = \angle XBD + \angle DCY = 180^{\circ} - \angle XOY,
so quadrilateral XOYKXOYK is inscribed. Because AZAZ is the diameter of the big circle, we have that ABBZAB \perp BZ, so O1O_1 is on line BZBZ. Similarly O2O_2 is on the line CZCZ. We prove that O1O_1, O2O_2 are on the circle OXKYOXKY. Indeed,
XO1K=2XBK=2(9012BOK)=180XOK, \angle XO_1K = 2\angle XBK = 2(90^{\circ} - \frac{1}{2}\angle BOK) = 180^{\circ} - \angle XOK,
Similarly for point O2O_2. Now we prove that point ZZ is on circle OXKYOXKY, which will finish the solution.

For this purpose we prove that O1ZC+O1KO2=180\angle O_1ZC + \angle O_1KO_2 = 180^{\circ}. Indeed,
O1KO2=O1KD+DKO2=90DBK+90DCK=180A=90BOC \angle O_1KO_2 = \angle O_1KD + \angle DKO_2 = 90^{\circ} - \angle DBK + 90^{\circ} - \angle DCK = 180^{\circ} - \angle A = 90^{\circ} - \angle BOC
=BZC=180O1ZC10th form10th form \begin{align*} &= \angle BZC = 180^{\circ} - \angle O_1ZC \\ &\text{10th form} \\ &\text{10th form} \end{align*}

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