Maths Olympiad Prep

Library / /2 of 24

Geometry Difficulty 7.7 National olympiad, round 2 Prove it Romania

Let ABCABC be a triangle with ABC=2ACB\angle ABC = 2 \cdot \angle ACB. Let XX and YY be the midpoints of arcs ABAB and BCBC (not containing CC and AA, respectively) of the circumcircle of triangle ABCABC. Let BLBL be the angle bisector of ABC\angle ABC, with LACL \in AC. Given that XLY=90\angle XLY = 90^\circ, determine the measures of the angles of triangle ABCABC.

Solution

From (1) and (2), we obtain that LBYLCY\triangle LBY \equiv \triangle LCY by the SSS criterion, and thus BLY=CLY\angle BLY = \angle CLY, which implies that LYLY is the angle bisector of BLC\angle BLC (3).
On the other hand, AX=BX\overline{AX} = \overline{BX} implies that ACX=XCB\angle ACX = \angle XCB, so XCXC is the angle bisector of LCB\angle LCB (4).

Let II be the intersection point of the segments LYLY and CXCX. From (3) and (4), we conclude that II is the incenter of triangle BLC\triangle BLC, hence BIBI is the angle bisector of LBC\angle LBC (5).
From the hypothesis, XLY=90\angle XLY = 90^\circ, and since LYLY is the angle bisector of BLC\angle BLC (from (3)), it follows that LXLX is the external angle bisector of BLC\angle BLC (6).
From (4) and (6), we deduce that XX is the excenter of triangle BLC\triangle BLC corresponding to vertex CC, hence XBXB is the external angle bisector of LBC\angle LBC. Combining this with (5), we obtain that XBBIXB \perp BI, i.e., XBI=90\angle XBI = 90^\circ (7).

Let DD be the intersection of BIBI with the circumcircle Γ\Gamma, and let α=IBC=IBL\alpha = \angle IBC = \angle IBL. Then ABL=LBC=2α\angle ABL = \angle LBC = 2\alpha. Since LBC=LCB=2α\angle LBC = \angle LCB = 2\alpha, we also have LCX=XCB=α\angle LCX = \angle XCB = \alpha. Therefore:
AD^=2ABD=2(ABL+LBD)=2(2α+α)=6α. \widehat{AD} = 2\angle ABD = 2(\angle ABL + \angle LBD) = 2(2\alpha + \alpha) = 6\alpha.
Also, AX^=2ACX=2α\widehat{AX} = 2\angle ACX = 2\alpha, so combining the two gives DX^=8α\widehat{DX} = 8\alpha.
On the other hand, DX^=2XBD=180\widehat{DX} = 2\angle XBD = 180^\circ, by (7). Combining these two relations gives 8α=1808\alpha = 180^\circ, hence ACB=2α=45\angle ACB = 2\alpha = 45^\circ, ABC=4α=90\angle ABC = 4\alpha = 90^\circ, and thus BAC=45\angle BAC = 45^\circ.

Want a route through all this instead of an archive? The track puts 2,000 problems in a working order, from AMC 10 level to the IMO shortlist.

Source: MathNet, licensed CC-BY-4.0. Statement and solution reproduced as published; topic and difficulty added by this site.