2016 AMC 10B Problem 19

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Concepts:similarityparallel linesrectangle

Difficulty rating: 1970

19.

Rectangle ABCDABCD has AB=5AB=5 and BC=4.BC=4. Point EE lies on AB\overline{AB} so that EB=1,EB=1, point GG lies on BC\overline{BC} so that CG=1,CG=1, and point FF lies on CD\overline{CD} so that DF=2.DF=2. Segments AG\overline{AG} and AC\overline{AC} intersect EF\overline{EF} at QQ and P,P, respectively. What is the value of PQEF?\dfrac{PQ}{EF}?

 316 ~\dfrac{\sqrt{3}}{16}

 213 ~\dfrac{\sqrt{2}}{13}

 982 ~\dfrac{9}{82}

 1091 ~\dfrac{10}{91}

 19 ~\dfrac19

Solution:

Observe that the value AE=ABEBAE = AB-EB=51 = 5-1=4.=4. Also, the value FC=DCDFFC = DC-DF =52= 5-2=3.=3. Then, since AEFC,AE \mid \mid FC, we know AEPCFP\triangle AEP \sim \triangle CFP by angle angle symmetry. Thus, PFPE=FCEA=34.\dfrac{PF}{PE} = \dfrac{FC}{EA} = \dfrac 34 . This makes PFEF=33+4=37.\dfrac{PF}{EF} = \dfrac{3}{3+4} = \dfrac 37.

Then, let XX be the extension of AGAG to meet CD.CD. Since two sides are parallel, we have ADDX=BGAB.\dfrac{AD}{DX} = \dfrac{BG}{AB}. Thus, 4DX=35\dfrac{4}{DX} = \dfrac{3}{5} DX=203.DX = \dfrac{20}3. This makes FX=2032=143.FX = \dfrac{20}3 - 2 = \dfrac{14}3 . Then, since AEFX,AE \mid \mid FX, we know AEQXFQ\triangle AEQ \sim \triangle XFQ by angle angle symmetry. Therefore, QFQE=FXEA=1434=76.\dfrac{QF}{QE} = \dfrac{FX}{EA} = \dfrac {\frac{14}{3}}4 = \dfrac 76. This makes QFEF=77+6=713.\dfrac{QF}{EF} = \dfrac{7}{7+6} = \dfrac 7{13}.

As such, QPEF=QFEFPFEF\dfrac{QP}{EF} = \dfrac{QF}{EF} - \dfrac{PF}{EF} =71337= \dfrac{7}{13} - \frac 37 =1091.= \dfrac{10}{91}.

Thus, the correct answer is D .

Problem 19 in Other Years