Mystic cevians 2

Geometry Level 5

Cevians A D , B E , C F AD, BE, CF are constructed in a triangle A B C ABC . Let the feet of perpendiculars on A D AD from B B and C C respectively be B a B_{a} and C a C_{a} , the feet of perpendiculars on B E BE from C C and A A respectively be C b C_{b} and A b A_{b} and the feet of perpendiculars on C F CF from A A and B B respectively be A c A_{c} and B c B_{c} .

Suppose:

B B a C C a = C C b A A b = A A c B B c = κ \large{\frac{BB_{a}}{CC_{a}}=\frac{CC_{b}}{AA_{b}}=\frac{AA_{c}}{BB_{c}}=\kappa}

It may be observed then, that the cevians shall be concurrent if and only if κ = 1 \kappa = 1 . Suppose κ \kappa differs from 1 1 by δ % \delta \% . Then the cevians shall meet pairwise at three different points P 1 , P 2 P_{1},P_{2} and P 3 P_{3} .

Determine for what value of δ \delta will the triangle P 1 P 2 P 3 P_{1}P_{2}P_{3} have an area Δ 2014 \frac{\Delta}{2014} where Δ \Delta is the area of triangle A B C ABC .

N o t e : \large{Note:} Report the rounded off value for δ \delta , i.e. the integer closest to δ \delta . Inspired by a popular problem, otherwise, original!


The answer is 4.

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1 solution

Aditya Kumar
Dec 27, 2014

To begin with, the ratio B B a C C a = κ = B D D C \large{\frac{BB_{a}}{CC_{a}} = \kappa = \frac{BD}{DC}}

This is from an elementary similiarity relationship between Δ B B a D \Delta BB_{a}D and Δ C C a D \Delta CC_{a}D . So, doing the same with all other ratios: we end up with the figure on the right with just the cevian incidence ratios on the edges shown.

Apply Menelaus' theorem (magnitudes) on Δ A B D \Delta ABD with transversal F C FC in order to determine the ratio A P 3 P 3 D \large{\frac{AP_{3}}{P_{3}D}} . Hence:

A F F B B C C D D P 3 P 3 A = 1 \large{\frac{AF}{FB}\frac{BC}{CD}\frac{DP_{3}}{P_{3}A} = 1}

A P 3 P 3 D = F B A F C D B C = κ 1 1 + κ 1 = κ ( κ + 1 ) \implies \large{\frac{AP_{3}}{P_{3}D} = \frac{FB}{AF}\frac{CD}{BC} = \frac{\kappa}{1}\frac{1+\kappa}{1}} = \kappa(\kappa+1)

So, A P 3 = κ ( κ + 1 ) κ ( κ + 1 ) + 1 × A C AP_{3} = \frac{\kappa(\kappa+1)}{\kappa(\kappa+1)+1}\times AC

Now, area of Δ A D C = 1 κ + 1 × Δ \Delta ADC = \frac{1}{\kappa+1}\times \Delta __ (1)

since, D C = 1 κ + 1 × B C DC = \frac{1}{\kappa+1}\times BC (area of triangles with a common height is proportional to the respective bases)

But, area of Δ A P 3 C = κ ( κ + 1 ) κ ( κ + 1 ) + 1 × \Delta AP_{3}C = \frac{\kappa(\kappa+1)}{\kappa(\kappa+1)+1}\times area of Δ A D C \Delta ADC __ (2)

since, A P 3 = κ ( κ + 1 ) κ ( κ + 1 ) + 1 × A C AP_{3} = \frac{\kappa(\kappa+1)}{\kappa(\kappa+1)+1}\times AC (area of triangles with a common height is proportional to the respective bases)

Combining (1) and (2),

area of Δ A P 3 C = κ κ ( κ + 1 ) + 1 × Δ \Delta AP_{3}C = \frac{\kappa}{\kappa(\kappa+1)+1}\times \Delta

The same relation holds for the other triangles : Δ C P 1 B \Delta CP_{1}B and Δ B P 2 A \Delta BP_{2}A .

So, we have :

Area of Δ P 1 P 2 P 3 = Δ ( A r e a Δ C P 1 B + A r e a Δ B P 2 A + A r e a Δ A P 3 C ) \Delta P_{1}P_{2}P_{3} = \Delta - (Area\Delta CP_{1}B+Area\Delta BP_{2}A+Area\Delta AP_{3}C)

= Δ 3 × κ κ ( κ + 1 ) + 1 × Δ = \Delta - \frac{3 \times \kappa}{\kappa(\kappa+1)+1}\times \Delta

= ( κ 1 ) 2 κ ( κ + 1 ) + 1 × Δ = \frac{(\kappa - 1)^2}{\kappa(\kappa+1)+1}\times \Delta

( Note : The famous case of κ = 2 \kappa =2 yields Δ 7 \frac{\Delta}{7} )

So, we need to solve: κ ( κ + 1 ) + 1 ( κ 1 ) 2 = 2014 \frac{\kappa(\kappa+1)+1}{(\kappa - 1)^2} = 2014

This becomes a quadratic equation and (as expected) the roots are reciprocals being : 1.039 ~1.039 and 0.962 ~ 0.962

Now, the percentage deviation from unity (for either number) ^* to the nearest integer is 4 % 4 \% .

Footnote:

^* It is interesting that the percentage deviation is the same for the two reciprocals. Indeed this so because : 1 1 + δ 1 δ \large{\frac{1}{1+\delta}} \approx 1-\delta for δ 1 \delta \ll 1

Very well put solution !

Venkata Karthik Bandaru - 6 years, 3 months ago

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