Maths Olympiad Prep

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Geometry Difficulty 5.2 AIME, harder Prove it Philippines

Problem:

Let ABCABC be a triangle such that the altitude from AA, the median from BB, and the internal angle bisector from CC meet at a single point. If BC=10BC=10 and CA=15CA=15, find AB2AB^{2}.

Solution

Solution:

Let DD be the foot of the AA-altitude, EE the midpoint of ACAC, and FF the foot of the CC-internal angle bisector. Then by Ceva's Theorem, we have
AFFBBDDCCEEA=1, \frac{AF}{FB} \cdot \frac{BD}{DC} \cdot \frac{CE}{EA}=1,
and so
baccosBbcosC=1, \frac{b}{a} \cdot \frac{c \cos B}{b \cos C}=1,
where we are using the shorthand BC=aBC=a, CA=bCA=b, AB=cAB=c. By cosine law, we know that
cosC=a2+b2c22ab \cos C=\frac{a^{2}+b^{2}-c^{2}}{2ab}
and
cosB=c2+a2b22ca. \cos B=\frac{c^{2}+a^{2}-b^{2}}{2ca}.
Substituting this into the equation, we obtain
a(a2+b2c2)=b(c2+a2b2) a\left(a^{2}+b^{2}-c^{2}\right)=b\left(c^{2}+a^{2}-b^{2}\right)
Solving for c2=AB2c^{2}=AB^{2} in this equation then gives the final answer, which is 205205.

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