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Geometry Difficulty 5.5 AIME, harder Prove it Estonia

The angles of a triangle are 22.522.5^\circ, 4545^\circ and 112.5112.5^\circ. Prove that inside this triangle there exists a point that is located on the median through one vertex, the angle bisector through another vertex and the altitude through the third vertex.

Solutions — 3

Solution 1

Let DD be the point of intersection of BCBC and median from vertex AA, EE be the point of intersection of angle bisector from vertex BB and ACAC, and FF be the point of intersection of altitude from vertex CC and ABAB (see fig. 6). As FBC=45\angle FBC = 45^\circ and CFB=90\angle CFB = 90^\circ, triangle FBCFBC is a right isosceles triangle with CF=FB|CF| = |FB|.

Figure 1
Figure 6

Let XX and YY be the points of intersection of the line that passes through point DD and is parallel to CFCF, with lines ACAC and AFAF, respectively. We have YAX=22.5\angle YAX = 22.5^\circ, XYA=90\angle XYA = 90^\circ and AXY=180YAXXYA=67.5\angle AXY = 180^\circ - \angle YAX - \angle XYA = 67.5^\circ. Therefore
XCD=180BCA=67.5=AXY=CXD, \angle XCD = 180^\circ - \angle BCA = 67.5^\circ = \angle AXY = \angle CXD,
due to which XD=CD=DB|XD| = |CD| = |DB|.
As line segments DYDY and CFCF are parallel, DYDY is the midsegment of triangle BCFBCF. Hence
XDDY=DBDY=CBCF=CBFB. \frac{|XD|}{|DY|} = \frac{|DB|}{|DY|} = \frac{|CB|}{|CF|} = \frac{|CB|}{|FB|}.
Let now KK be the point of intersection of BEBE and CFCF. The angle bisector property gives that CKKF=CBFB\frac{|CK|}{|KF|} = \frac{|CB|}{|FB|}, so XDDY=CKKF\frac{|XD|}{|DY|} = \frac{|CK|}{|KF|}, from which FAK=YAD\angle FAK = \angle YAD. Therefore also ADAD passes through KK, QED.

Figure 2

Figure 2
Figure 7

Solution 2

Similarly to the previous solution we pick triangle ABCABC, mark points DD, EE and FF and show that CF=FB|CF| = |FB|. Additionally notice that ABE=ABC2=452=22.5=BAE\angle ABE = \frac{\angle ABC}{2} = \frac{45^\circ}{2} = 22.5^\circ = \angle BAE, which gives BE=AE|BE| = |AE|.
Let now ZZ be a point on BCBC such that BEZ=90\angle BEZ = 90^\circ (see fig. 7). Then ZBE=ABC2=22.5=CAF\angle ZBE = \frac{\angle ABC}{2} = 22.5^\circ = \angle CAF and BEZ=90=AFC\angle BEZ = 90^\circ = \angle AFC, hence triangles BEZBEZ and AFCAFC are
EZC=180ZBEBEZ=67.5=180BCA=ECZ, \angle EZC = 180^\circ - \angle ZBE - \angle BEZ = 67.5^\circ = 180^\circ - \angle BCA = \angle ECZ,
which gives EZ=EC|EZ| = |EC|. Thus AEEC=BEEZ=AFFC=AFFB\frac{|AE|}{|EC|} = \frac{|BE|}{|EZ|} = \frac{|AF|}{|FC|} = \frac{|AF|}{|FB|}. Therefore AFFBBDDC=CEEA=1\frac{|AF|}{|FB|} \cdot \frac{|BD|}{|DC|} = \frac{|CE|}{|EA|} = 1 and Ceva's theorem gives that ADAD, BEBE and CFCF intersect in one point.

Solution 3

Similarly to first solution we choose triangle ABCABC, mark points DD, EE, FF and YY and show that CF=FB|CF| = |FB|.
Let the points of intersection of CFCF with angle bisector BEBE and median ADAD be K1K_1 and K2K_2, respectively. Let h1=FK1h_1 = |FK_1| and h2=FK2h_2 = |FK_2| and in addition let u=AFu = |AF| and v=CF=FBv = |CF| = |FB|. Let us show that h1=h2h_1 = h_2, then K1=K2K_1 = K_2. Using the similarity of triangles BFK1BFK_1 and CFACFA, similarity of triangles BCK1BCK_1 and ABCABC and similarity of triangles AFK2AFK_2 and AYDAYD, we get the equalities
h1v=vu,vh12v=2vu+v,h2u=12vu+12v \frac{h_1}{v} = \frac{v}{u}, \quad \frac{v-h_1}{\sqrt{2}v} = \frac{\sqrt{2}v}{u+v}, \quad \frac{h_2}{u} = \frac{\frac{1}{2}v}{u+\frac{1}{2}v}
which imply v2u=h1=(uv)vu+v\frac{v^2}{u} = h_1 = \frac{(u-v)v}{u+v} and h2=uv2u+vh_2 = \frac{uv}{2u+v}. From equation v2u=(uv)vu+v\frac{v^2}{u} = \frac{(u-v)v}{u+v} we get 2u+v=u2v2u + v = \frac{u^2}{v}, therefore h2=uv2u+v=v2u=h1h_2 = \frac{uv}{2u+v} = \frac{v^2}{u} = h_1.

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