Triangle, Hard from every angle!!

Geometry Level 2

In the above figure, in ∆ABC, BC is extended to E such that CE= 1 2 \frac{1}{2} BC. IF D is the mid point of AC and ED cuts AB at F, Find the value of A B A F \frac{AB}{AF}


The answer is 4.

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2 solutions

Aryan Sanghi
Sep 14, 2018

Construction: Through D, draw a line parallel to AB cutting AE at G and BE at H. Now, through C, draw a line parallel to BA cutting AE at I and ED at J.


Solution:

In ∆EFB

FD= 1 2 \frac{1}{2} ED. (Basic Proportionality Theorem)

In ∆CAB

CH=BH= 1 2 \frac{1}{2} BC (Mid-Point Theorem)

=> HC=EC

In ∆EDH

EJ=DJ= 1 2 \frac{1}{2} ED. (Mid-Point Theorem)

=> DJ=DF

=> ∆AFD is congruent to ∆CJD

=> AF=JC=a

In ∆AEF

IJ= 1 3 \frac{1}{3} AF. (Basic Proportionality Theorem)

c= 1 3 \frac{1}{3} a

In ∆AEB

CI= 1 3 \frac{1}{3} BA= b 3 \frac{b}{3} . (Basic Proportionality Theorem)

b 3 \frac{b}{3} =c+a (CI=a+c)

b 3 \frac{b}{3} = a 3 \frac{a}{3} +a

a= b 4 \frac{b}{4}

b a \frac{b}{a} =4

Shashi Kamal
Sep 14, 2018

Use Mass Point Geometry.

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