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Let $\displaystyle AD$, $\displaystyle BE$, $\displaystyle CF$ be three cevians concurrent at $\displaystyle P$ inside the $\displaystyle \Delta ABC$.

Prove or disprove that:

$$\displaystyle \dfrac{AD}{AP} + \dfrac{BE}{BP} + \dfrac{CF}{CP} \ge \dfrac{9}{2}$$

1 Answers 1

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The inequality is true!

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It can be shown that (see proof at the end of the answer)

$$\displaystyle \frac{PD}{AD} + \frac{PE}{BE} + \frac{PF}{CF} = 1$$

Note that this implies that

$$\displaystyle \frac{AP}{AD} + \frac{BP}{BE} + \frac{CP}{CF} = 2$$

as $\displaystyle 1 - \frac{PD}{AD} = \frac{AP}{AD}$ etc.

Now we have the inequality (easily shown using $\text{AM} \ge \text{GM}$) that

$$\displaystyle (a_1 + a_2 + a_3)(\frac{1}{a_1} + \frac{1}{a_2} + \frac{1}{a_3}) \ge 9$$

This shows that

$$\displaystyle (\frac{AP}{AD} + \frac{BP}{BE} + \frac{CP}{CF})(\frac{AD}{AP} + \frac{BE}{BP} + \frac{CF}{CP}) \ge 9$$

and so

$$\displaystyle 2(\frac{AD}{AP} + \frac{BE}{BP} + \frac{CF}{CP}) \ge 9$$

i.e.

$$\displaystyle \frac{AD}{AP} + \frac{BE}{BP} + \frac{CF}{CP} \ge \frac{9}{2}$$

Note that the equality occurs only when $\displaystyle \frac{AP}{AD} = \frac{BP}{BE} = \frac{CP}{CF} = \frac{2}{3}$, which implies that $\displaystyle P$ is the centroid.


Proof

Let us try showing that

$$\displaystyle \frac{PD}{AD} + \frac{PE}{BE} + \frac{PF}{CF} = 1$$

Consider the figure (repeated from above for convenience).

alt text

Note, if you are worried about acute triangle vs obtuse etc, a simple affine transformation will do to transform the triangle into an equilateral triangle.

Let X be the foot of perpendicular from A to BC and Y be the foot of the perpendicular from P to BC.

$\displaystyle \triangle AXD$ and $\triangle PYD$ are similar and thus

$\displaystyle \frac{PY}{AX} = \frac{PD}{AD}$.

Now $\displaystyle \frac{PY}{AX} = \frac{|\triangle PBC|}{|\triangle ABC|}$

where $\displaystyle |\triangle MNO|$ is the area of $\displaystyle \triangle MNO$.

Thus $\displaystyle \frac{PD}{AD} = \frac{|\triangle PBC|}{|\triangle ABC|}$

Similarly

$\displaystyle \frac{PE}{BE} = \frac{|\triangle PAC|}{|\triangle ABC|}$

$\displaystyle \frac{PF}{CF} = \frac{|\triangle PAB|}{|\triangle ABC|}$

Adding gives us

$$\displaystyle \frac{PD}{AD} + \frac{PE}{BE} + \frac{PF}{CF} = \frac{|\triangle PAB| + |\triangle PAC| + |\triangle PBC|}{|\triangle ABC|} = \frac{|\triangle ABC|}{|\triangle ABC|} = 1$$

  • 0
    Beautiful! Thank you so much.2010-12-04
  • 0
    @Robert: You are welcome! It was a nice problem, btw. Where did you get it from?2010-12-04
  • 0
    From a book as an exercise.2010-12-04