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Geometry Difficulty 5.8 AIME, harder Prove it Greece

From the vertex AA of an equilateral triangle ABCABC we draw the ray AxAx which intersects the side BΓB\Gamma at Δ\Delta. On AxAx we consider a point EE such that BA=BEBA = BE. Find the angle AE^ΓA\hat{E}\Gamma.

Solutions — 2

Solution 1

Since BA=BΓ=BEBA = B\Gamma = BE, point BB is the circumcenter of the triangle AΓEA\Gamma E. The angle AE^ΓA\hat{E}\Gamma is inscribed to the circle C(B,BA)C(B, BA) and so:
AE^Γ=12AB^Γ=30. A\hat{E}\Gamma = \frac{1}{2} A\hat{B}\Gamma = 30^{\circ}.

Solution 2

From BA=BEBA = BE and BA=BΓBA = B\Gamma, we have BΓ=BEB\Gamma = BE, and the triangle BΓEB\Gamma E is isosceles. We draw the altitude from BB, let BZBZ, ZΓEZ \in \Gamma E.

Figure 1

Then BZBZ is also median and bisector of the angle EBΓE B \Gamma of the triangle BΓEB\Gamma E. Let BZBZ meet AEAE at KK. Then the triangles BKΓBK\Gamma and BKEBKE are equal because they have:

Figure 2

BΓ=BE, BK common side and KBΓ=KBE. B\Gamma = BE,\ BK \text{ common side and } KB\Gamma = KBE.
Therefore we have: BΓK=BE^KB\Gamma K = B\hat{E}K and since
BE^K=BA^K it follows that BΓK=BA^K. B\hat{E}K = B\hat{A}K \text{ it follows that } B\Gamma K = B\hat{A}K.
Thus quadrilateral ABKΓABK\Gamma is inscribable, and hence:
ZKE=BKA=BΓA=60 ZKE = BKA = B\Gamma A = 60^{\circ}
Finally we get:
AE^Γ=KE^Z=90EKZ=9060=30. A\hat{E}\Gamma = K\hat{E}Z = 90^{\circ} - EKZ = 90^{\circ} - 60^{\circ} = 30^{\circ}.

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