Cevians are constructed in a triangle . Let the feet of perpendiculars on from and respectively be and , the feet of perpendiculars on from and respectively be and and the feet of perpendiculars on from and respectively be and .
Suppose:
It may be observed then, that the cevians shall be concurrent if and only if . Suppose differs from by . Then the cevians shall meet pairwise at three different points and .
Determine for what value of will the triangle have an area where is the area of triangle .
Report the rounded off value for , i.e. the integer closest to . Inspired by a popular problem, otherwise, original!
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To begin with, the ratio C C a B B a = κ = D C B D
This is from an elementary similiarity relationship between Δ B B a D and Δ C C 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 with transversal F C in order to determine the ratio P 3 D A P 3 . Hence:
F B A F C D B C P 3 A D P 3 = 1
⟹ P 3 D A P 3 = A F F B B C C D = 1 κ 1 1 + κ = κ ( κ + 1 )
So, A P 3 = κ ( κ + 1 ) + 1 κ ( κ + 1 ) × A C
Now, area of Δ A D C = κ + 1 1 × Δ __ (1)
since, D C = κ + 1 1 × B C (area of triangles with a common height is proportional to the respective bases)
But, area of Δ A P 3 C = κ ( κ + 1 ) + 1 κ ( κ + 1 ) × area of Δ A D C __ (2)
since, A P 3 = κ ( κ + 1 ) + 1 κ ( κ + 1 ) × A C (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 + 1 ) + 1 κ × Δ
The same relation holds for the other triangles : Δ C P 1 B and Δ B P 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 )
= Δ − κ ( κ + 1 ) + 1 3 × κ × Δ
= κ ( κ + 1 ) + 1 ( κ − 1 ) 2 × Δ
( Note : The famous case of κ = 2 yields 7 Δ )
So, we need to solve: ( κ − 1 ) 2 κ ( κ + 1 ) + 1 = 2 0 1 4
This becomes a quadratic equation and (as expected) the roots are reciprocals being : 1 . 0 3 9 and 0 . 9 6 2
Now, the percentage deviation from unity (for either number) ∗ to the nearest integer is 4 % .
Footnote:
∗ It is interesting that the percentage deviation is the same for the two reciprocals. Indeed this so because : 1 + δ 1 ≈ 1 − δ for δ ≪ 1