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Geometry/Triangles

Question

MCQ} In triangle ABCABC, point DD lies on side BCBC such that BD:DC=2:3BD:DC=2:3. Point EE lies on segment ADAD such that AE:ED=3:2AE:ED=3:2. The line through BB and EE, when extended, meets side ACAC at point FF. The ratio AF:FCAF:FC equals:

Options

A

3:53:5

B

3:43:4

C

9:109:10

D

2:32:3

Answer

(A) 3:53:5.

Detailed solution

Place AA at the origin and use vectors. Let b=AB,c=AC.\vec b = \vec{AB}, \vec c = \vec{AC}.

DD on BCBC with BD:DC=2:3BD:DC=2:3: AD=3b+2c5.\vec{AD} = \dfrac{3\vec b + 2\vec c}{5}.

EE on ADAD with AE:ED=3:2AE:ED=3:2: AE=35AD=3(3b+2c)25=9b+6c25.\vec{AE} = \dfrac{3}{5}\vec{AD} = \dfrac{3(3\vec b + 2\vec c)}{25} = \dfrac{9\vec b + 6\vec c}{25}.

Parametrise line BEBE: r=b+t(AEb)=b+t ⁣(9b+6c25b)=b ⁣(116t25)+c6t25.\vec r = \vec b + t(\vec{AE} - \vec b) = \vec b + t\!\left(\dfrac{9\vec b + 6\vec c}{25} - \vec b\right) = \vec b\!\left(1 - \dfrac{16t}{25}\right) + \vec c\cdot\dfrac{6t}{25}.

For this point to lie on line ACAC (i.e., r=sc\vec r = s\vec c), the coefficient of b\vec b must vanish:

$

1 - \dfrac{16t}{25} = 0 ;\Longrightarrow; t = \dfrac{25}{16}.

$

Then r=c6252516=38c.\vec r = \vec c\cdot\dfrac{6}{25}\cdot\dfrac{25}{16} = \dfrac{3}{8}\vec c.

So AF=38c\vec{AF} = \dfrac{3}{8}\vec c, meaning AF:AC=3:8AF:AC = 3:8, hence

$

AF:FC = 3:5.

$

Cross-check by mass points: Place masses: at CC mass 22, at BB mass 33 so DD has mass 55 (since BD:DC=2:3BD:DC=2:3). Place mass at AA such that AE:ED=3:2AE:ED=3:2, i.e., mass at AA should be in ratio 2:32:3 with mass at DD, so mass at AA is 22 and mass at DD is 33 (consistent up to scale; rescale DD's mass from 55 to 33 by dividing by 5/35/3; equivalently, multiply AA's mass by 5/35/3). To keep arithmetic clean: mass at A=25=10A=2\cdot 5 = 10, at D=35=15D = 3\cdot 5 = 15, so at BB effective mass =(3/5)15=9= (3/5)\cdot 15 = 9 and at CC effective mass =(2/5)15=6.= (2/5)\cdot 15 = 6. Then on ACAC, masses 1010 at AA and 66 at CC, so FF divides ACAC in ratio 6:10=3:56:10 = 3:5 from AA. \checkmark

Why this is CAT-level: The natural temptation is to use Menelaus on ADC\triangle ADC with transversal BB--EE--FF, but choosing the right triangle and getting the signed ratios correct is error-prone under time pressure. Vector / mass-point methods are faster and trap-resistant. Wrong option (C) 9:109:10 corresponds to a common Menelaus sign error.

Answer: (A) 3:53:5.