2019 AMC 8 Problem 24

Attempt Problem 24 of the 2019 AMC 8 below, then check your answer against the video solution and professionally curated solution from LIVE by Po-Shen Loh. You can also try the full timed exam, view all 2019 AMC 8 solutions, or check the answer key.

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24.

In triangle ABC,ABC, point DD divides side AC‾\overline{AC} so that AD:DC=1:2.AD : DC = 1 : 2. Let EE be the midpoint of BD‾\overline{BD} and let FF be the point of intersection of line BCBC and line AE.AE. Given that the area of △ABC\triangle ABC is 360,360, what is the area of △EBF?\triangle EBF?
2424
3030
3232
3636
4040
Answer: B
Concepts:area ratiotriangle area
Small hint:
Use area ratios from bases on the same line
Big hint:
Let x=[EBF]x = [EBF] and use [AFD]:[DFC]=1:2[AFD] : [DFC] = 1 : 2
Video solution:
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Written solution:
Since AD:DC=1:2,AD : DC = 1 : 2, triangles ABDABD and DBCDBC have areas in the ratio 1:2.1 : 2. Thus [ABD]=120[ABD] = 120 and [DBC]=240.[DBC] = 240.
Because EE is the midpoint of BD,BD, triangles ABEABE and AEDAED each have area 60.60. Let x=[EBF].x = [EBF]. Then [DEF]=x[DEF] = x as well, since BE=EDBE = ED and both triangles have their third vertex on line AF.AF.
Segment DFDF splits △DBC,\triangle DBC, so [DFC]=240−2x.[DFC] = 240 - 2x. Also, △ADF\triangle ADF and △DFC\triangle DFC have bases ADAD and DCDC on the same line, so their areas are in the ratio 1:2.1 : 2.
Therefore 60+x240−2x=12. \dfrac{60 + x}{240 - 2x} = \dfrac{1}{2}. Solving gives 120+2x=240−2x,120 + 2x = 240 - 2x, so x=30.x = 30.
Thus, the correct answer is B.
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