Maths Olympiad Prep

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Geometry Difficulty 5.1 AIME, harder Prove it Ukraine

Find the angles of at least one triangle, one bisector of which is twice bigger than another.

Solution

Let's find an isosceles triangle. Let's pretend that the vertex angle is obtuse. Then let's denote the base angle as 2α2\alpha (Fig. 49). Then we can easily find values of some angles (Fig. 49):
ALC=π3α,ALB=3α,ABC=π4α. \angle ALC = \pi - 3\alpha, \quad \angle ALB = 3\alpha, \quad \angle ABC = \pi - 4\alpha.

Let's denote the bisection and the sides as follows: AL=lAL = l, AD=hAD = h, AB=bAB = b, then 2h=l2h = l.
Then h=bsin2αh = b \sin 2\alpha. By the law of sines for ABL\triangle ABL: bsin3α=lsin4α\frac{b}{\sin 3\alpha} = \frac{l}{\sin 4\alpha}. So, 2h=2bsin2α=bsin4αsin3α2h = 2b \sin 2\alpha = \frac{b \sin 4\alpha}{\sin 3\alpha}
2sin2αsin3α=2sin2αcos2αsin3α=cos2α\Rightarrow 2 \sin 2\alpha \sin 3\alpha = 2 \sin 2\alpha \cos 2\alpha \Rightarrow \sin 3\alpha = \cos 2\alpha.
The answer of this equality is pretty clear: α=18\alpha = 18^\circ.

Answer: 3636^\circ, 3636^\circ, 108108^\circ.

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Source: MathNet, licensed CC-BY-4.0. Statement and solution reproduced as published; topic and difficulty added by this site.