A geometry problem by A Former Brilliant Member

Geometry Level pending

In parallelogram A B C D ABCD shown above, point E E lies on diagonal A C AC . Line B E BE extended meets C D CD at F F and A D AD at G G . Given that A D = 10 , D G = 4 AD=10,DG=4 and B G = 11 , BG=11, find the length of E F EF correct to four decimal places.


The answer is 3.2738.

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1 solution

Since B A G F D G \triangle BAG \sim \triangle FDG , we have, G F 11 = 4 14 \dfrac{GF}{11}=\dfrac{4}{14} .

G F = 4 ( 11 ) 14 = 22 7 GF=\dfrac{4(11)}{14}=\dfrac{22}{7}

Since E B C E G A \triangle EBC \sim \triangle EGA , we have, B E E G = 10 14 \dfrac{BE}{EG}=\dfrac{10}{14} .

B E 11 B E = 10 14 \dfrac{BE}{11-BE}=\dfrac{10}{14} \color{#D61F06}\implies 14 B E = 10 ( 11 B E ) 14BE=10(11-BE) \color{#D61F06}\implies 24 B E = 110 24BE=110 \color{#D61F06}\implies B E = 55 12 BE=\dfrac{55}{12}

Finally,

E F = 11 B E G F = 11 55 12 22 7 = 275 84 = EF=11-BE-GF=11-\dfrac{55}{12}-\dfrac{22}{7}=\dfrac{275}{84}= 3.2738 \boxed{3.2738}

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