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Geometry Difficulty 6.2 National olympiad Prove it Estonia

The internal angle bisectors at vertices BB and CC of triangle ABCABC intersect the circumcircle of triangle ABCABC at EE and FF, respectively. Given that BE=CF0BE = CF \neq 0, may we be sure that triangle ABCABC is isosceles?

Solutions — 2

Solution 1

Let ABC=30\angle ABC = 30^\circ and BCA=90\angle BCA = 90^\circ. Let OO be the circumcenter of triangle ABCABC; it is also the midpoint of the hypotenuse ABAB. We have OAC=BAC=60\angle OAC = \angle BAC = 60^\circ, and since AO=COAO = CO, it follows that OCA=60\angle OCA = 60^\circ (Fig. 30). Therefore, FCO=60902=15\angle FCO = 60^\circ - \frac{90^\circ}{2} = 15^\circ. Let CC' be the reflection of vertex CC over the point OO (Fig. 31); then FCC=15\angle FCC' = 15^\circ as well. On the other hand, ABE=302=15\angle ABE = \frac{30}{2}^\circ = 15^\circ. In conclusion, we see that the arcs AEAE and FCFC' subtend equal inscribed angles on the circumcircle of triangle ABCABC, hence the corresponding arcs are equal. Since ABAB and CCCC' are diameters, the remaining arcs BEBE and CFCF are also equal. Therefore, the corresponding chords BE=CFBE = CF are equal. Thus, the equality BE=CFBE = CF can hold even in a non-isosceles triangle.

Figure 1
Fig. 30
Figure 2
Fig. 31

Solution 2

Let CAB=60\angle CAB = 60^\circ and AC<ABAC < AB, and let OO be the circumcenter of triangle ABCABC (Fig. 32). Then BOC=260=120\angle BOC = 2 \cdot 60^\circ = 120^\circ. Thus, COE+EOA+AOF+FOB=360120=240\angle COE + \angle EOA + \angle AOF + \angle FOB = 360^\circ - 120^\circ = 240^\circ. Since COE=EOA\angle COE = \angle EOA and AOF=FOB\angle AOF = \angle FOB, we have EOA+AOF=2402=120\angle EOA + \angle AOF = \frac{240^\circ}{2} = 120^\circ. Consequently, EOF=BOC\angle EOF = \angle BOC, from which it follows that
COF=COE+EOF=BOC+COE=BOE. \angle COF = \angle COE + \angle EOF = \angle BOC + \angle COE = \angle BOE.
In conclusion, we see that the central angles subtended by the shorter arcs BEBE and CFCF of the circumcircle of triangle ABCABC are equal, hence the corresponding arcs are equal. Therefore, the corresponding chords BEBE and CFCF are equal. Thus, the equality BE=CFBE = CF can hold in a non-isosceles triangle.

Figure 3
Fig. 32

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