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Geometry Difficulty 5.7 AIME, harder Prove it North Macedonia

Calculate the angles in the triangle ABCABC, if the angle between the altitude from CC and the bisector of the angle ACBACB is 9090^\circ, and the angle between the bisectors of the exterior angles at the vertices AA and BB is 6161^\circ.

Solution

Let us denote with α,β,γ\alpha, \beta, \gamma the angles in the triangle ABCABC at the vertices A,B,CA, B, C correspondently and with α1,β1,γ1\alpha_1, \beta_1, \gamma_1 the correspondent exterior angles. Let DD be the base of the altitude from CC, EE be the base of the bisector of the angle ACBACB and FF be the intersection of the bisectors AYAY and BNBN of the exterior angles at the vertices AA and BB correspondently. Then DCE=90\angle DCE = 90^\circ, AFB=61\angle AFB = 61^\circ, XAY=YAC=α12\angle XAY = \angle YAC = \frac{\alpha_1}{2}, CBN=NBM=β12\angle CBN = \angle NBM = \frac{\beta_1}{2} and ACE=ECB=γ12\angle ACE = \angle ECB = \frac{\gamma_1}{2}.

For the angles in the triangle ABFABF we have that
Figure 1

FAB=XAY=α12\angle FAB = \angle XAY = \frac{\alpha_1}{2} and ABF=NBM=β12\angle ABF = \angle NBM = \frac{\beta_1}{2} as opposite angles. Now we obtain α12+β12+61=180\frac{\alpha_1}{2} + \frac{\beta_1}{2} + 61^\circ = 180^\circ (because the sum of the angles in every triangle is 180180^\circ). Hence α1+β1=238\alpha_1 + \beta_1 = 238^\circ. Because α1=180α\alpha_1 = 180^\circ - \alpha and β1=180β\beta_1 = 180^\circ - \beta if substitute in the previous equality we get 180α+180β=238180^\circ - \alpha + 180^\circ - \beta = 238^\circ. Hence α+β=122\alpha + \beta = 122^\circ. γ=180(α+β)=180122=58\gamma = 180^\circ - (\alpha + \beta) = 180^\circ - 122^\circ = 58^\circ.

From the right-angled triangle ADCADC we have
α=90ACD=90(γ2CDE)=90(5829)=70. \alpha = 90^\circ - \angle ACD = 90^\circ - (\frac{\gamma}{2} - \angle CDE) = 90^\circ - (\frac{58^\circ}{2} - 9^\circ) = 70^\circ.
Now we get that β=122α=12270=52\beta = 122^\circ - \alpha = 122^\circ - 70^\circ = 52^\circ.

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Source: MathNet, licensed CC-BY-4.0. Statement reproduced verbatim; metadata (topic, difficulty) added by this project.