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Geometry Difficulty 6.3 National olympiad Prove it Estonia

The altitudes of an acute-angled triangle ABCABC intersect at point HH. The tangent at point AA to the circumcircle of triangle AHBAHB intersects the line CHCH at point KK. The tangent at point AA to the circumcircle of triangle AHCAHC intersects the line BHBH at point LL. Prove that the points BB, CC, KK, LL lie on the same circle.

Solutions — 2

Solution 1

Denote CAB=α\angle CAB = \alpha, ABC=β\angle ABC = \beta, and BCA=γ\angle BCA = \gamma. By tangency, KAH=ABH=90α\angle KAH = \angle ABH = 90^\circ - \alpha (Fig. 41). Since ACH=90α\angle ACH = 90^\circ - \alpha as well, triangles AHCAHC and KHAKHA are similar. Consequently, the corresponding third angles are equal, i.e., AKH=CAH=90γ\angle AKH = \angle CAH = 90^\circ - \gamma. Similarly, we get LAH=90α\angle LAH = 90^\circ - \alpha and ALH=90β\angle ALH = 90^\circ - \beta. Thus, KAL=KAH+LAH=1802α\angle KAL = \angle KAH + \angle LAH = 180^\circ - 2\alpha, with AHAH bisecting the angle KALKAL.

On the other hand,
KHL=CHB=180BCHCBH=180(90γ)(90β)=β+γ=180α. \begin{align*} \angle KHL &= \angle CHB = 180^\circ - \angle BCH - \angle CBH \\ &= 180^\circ - (90^\circ - \gamma) - (90^\circ - \beta) \\ &= \beta + \gamma = 180^\circ - \alpha. \end{align*}
Now note (Fig. 42) that for the incenter II of triangle AKLAKL,
KIL=180IKLILK=180AKL2ALK2=180AKL+ALK2=180180KAL2=180180(1802α)2=180α, \begin{align*} \angle KIL &= 180^\circ - \angle IKL - \angle ILK = 180^\circ - \frac{\angle AKL}{2} - \frac{\angle ALK}{2} \\ &= 180^\circ - \frac{\angle AKL + \angle ALK}{2} \\ &= 180^\circ - \frac{180^\circ - \angle KAL}{2} = 180^\circ - \frac{180^\circ - (180^\circ - 2\alpha)}{2} = 180^\circ - \alpha, \end{align*}
so KIL=KHL\angle KIL = \angle KHL. Since points HH and II lie on the same side of line KLKL, points KK, HH, II, and LL lie on the same circle. Given that both HH and II lie on the angle bisector AHAH of angle KALKAL, which intersects the chord KLKL, it follows that H=IH = I. Consequently, LKH=AKH=90γ\angle LKH = \angle AKH = 90^\circ - \gamma and KLH=ALH=90β\angle KLH = \angle ALH = 90^\circ - \beta. In addition, we have HLK=90β=HCB\angle HLK = 90^\circ - \beta = \angle HCB, from which it follows that points BB, CC, KK, LL lie on the same circle.

Solution 2

As in Solution 1, we note that triangles AHCAHC and KHAKHA are similar, hence HAHK=HCHA\frac{HA}{HK} = \frac{HC}{HA}, or HA2=HKHCHA^2 = HK \cdot HC. Similarly, triangles AHBAHB and LHALHA are similar, giving HA2=HLHBHA^2 = HL \cdot HB. Therefore, HBHL=HCHKHB \cdot HL = HC \cdot HK, from which it follows that points BB, CC, KK, LL lie on the same circle.

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