An isosceles obtuse triangle

Geometry Level pending

A B C \triangle ABC is an isosceles obtuse triangle with A B = A C \overline{AB} = \overline{AC} and A = 13 5 \angle A = 135^{\circ} . Its incircle, circle O O has a radius of 10 10 . Line segment B O BO is extended to meet A C AC at D D . Find A D + B D \overline{AD} + \overline{BD} .

Figure drawn to scale


The answer is 88.366.

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1 solution

Since A B C \triangle ABC is isosceles with A = 13 5 \angle A = 135^\circ , then B = C = 22. 5 \angle B = \angle C = 22.5^\circ . Let A E AE be the altitude of A B C \triangle ABC which passes through center O O of the incircle. Then O B OB bisects B \angle B and O B E = O B A = 11.2 5 \angle OBE = \angle OBA = 11.25^\circ .

We have B E = 10 tan 11.2 5 BE = \dfrac {10}{\tan 11.25^\circ} and A B = B E sin 22. 5 = 10 tan 11.2 5 sin 22. 5 54.41553054 AB = \dfrac {BE}{\sin 22.5^\circ} = \dfrac {10}{\tan 11.25^\circ \sin 22.5^\circ} \approx 54.41553054 .

By sine rule we have A B sin A D B = A D sin A B D = B D sin B A D \dfrac {AB}{\sin \angle ADB} = \dfrac {AD}{\sin \angle ABD} = \dfrac {BD}{\sin \angle BAD} , then

A D = A B sin 11.2 5 sin 33.7 5 19.10819325 B D = A B sin 13 5 sin 33.7 5 69.25783342 A D + B D 88.4 \begin{aligned} AD & = \frac {AB \cdot \sin 11.25^\circ}{\sin 33.75^\circ} \approx 19.10819325 \\ BD & = \frac {AB \cdot \sin 135^\circ}{\sin 33.75^\circ} \approx 69.25783342 \\ \implies AD+BD & \approx \boxed{88.4} \end{aligned}

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