Maths Olympiad Prep

Library / /338 of 520

Geometry Difficulty 5.5 AIME, harder Find the answer

Example 6. In ABC\triangle \mathrm{ABC}, it is known that AB>AC,P\mathrm{AB}>\mathrm{AC}, \mathrm{P} is any point on BC\mathrm{BC}, the symmetric point of P\mathrm{P} with respect to AB\mathrm{AB} is E\mathrm{E}, and the symmetric point of P\mathrm{P} with respect to AC\mathrm{AC} is F\mathrm{F}. When is the area of AEF\triangle \mathrm{AEF} the smallest (or largest)?

A number or a short expression. Spacing and $ signs are ignored.

Solution

From the given conditions,
(As shown in Figure 7),
EAB=PAB,FAC \begin{array}{l} \angle \mathrm{EAB} \\ =\angle \mathrm{PAB}, \\ \angle \mathrm{FAC} \end{array}
=PAC, hence EAP=2PAB,FAP=2PAC.EAF=EAP+FAP=PAB+PAC=8BAC= a constant, then SAEF=12AEAFsinEAF=12AP2 \begin{array}{l} =\angle \mathrm{PAC}, \text{ hence } \\ \angle \mathrm{EAP}=2 \angle \mathrm{PAB}, \angle \mathrm{FAP}=2 \angle \mathrm{PAC}. \\ \therefore \angle \mathrm{EAF}=\angle \mathrm{EAP}+\angle \mathrm{FAP}=\angle \mathrm{PAB} \\ +\angle \mathrm{PAC}=8 \angle B A C=\text{ a constant, then } \mathrm{S}_{\triangle A E F} \\ =\frac{1}{2} \mathrm{AE} \cdot \mathrm{AF} \sin \angle \mathrm{EAF}=\frac{1}{2} \mathrm{AP}^{2} \\ \end{array}
. sin2BAC\sin 2 \angle B A C. To minimize SAFF\mathrm{S}_{\triangle \mathrm{AFF}}, it is only necessary for AP\mathrm{AP} to be the smallest, so draw PPBC\mathrm{PP} \perp \mathrm{BC} through A\mathrm{A}, with P\mathrm{P} being the foot of the perpendicular, and point P\mathrm{P} is the required point; similarly, to maximize SAEF\mathrm{S}_{\triangle A E F}, it is only necessary for AP\mathrm{AP} to be the largest, so when point P\mathrm{P} coincides with point B\mathrm{B}, SAEF\mathrm{S}_{\triangle A E F} is the largest.

Want a route through all this instead of an archive? The track puts 2,000 problems in a working order, from AMC 10 level to the IMO shortlist.

Source: NuminaMath-1.5, licensed Apache-2.0. Statement and solution reproduced as published; topic and difficulty added by this site.