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Geometry Difficulty 7.5 National Olympiad, round 2 Prove it India

Problem:

Let ABCABC be a triangle. An interior point PP of ABCABC is said to be good if we can find exactly 2727 rays emanating from PP intersecting the sides of the triangle ABCABC such that the triangle is divided by these rays into 2727 smaller triangles of equal area. Determine the number of good points for a given triangle ABCABC.

Solution

Solution:

Let PP be a good point. Let l,m,nl, m, n be respectively the number of parts the sides BC,CA,ABBC, CA, AB are divided by the rays starting from PP. Note that a ray must pass through each of the vertices of the triangle ABCABC; otherwise we get some quadrilaterals.

Let h1h_1 be the distance of PP from BCBC. Then h1h_1 is the height for all the triangles with their bases on BCBC. Equality of areas implies that all these bases have equal length. If we denote this by xx, we get lx=al x = a. Similarly, taking yy and zz as the lengths of the bases of triangles on CACA and ABAB respectively, we get my=bm y = b and nz=cn z = c. Let h2h_2 and h3h_3 be the distances of PP from CACA and ABAB respectively. Then

h1x=h2y=h3z=2Δ27 h_1 x = h_2 y = h_3 z = \frac{2 \Delta}{27}

where Δ\Delta denotes the area of the triangle ABCABC. These lead to

h1=2Δ27la,h2=2Δ27mb,h3=2Δ27nc h_1 = \frac{2 \Delta}{27} \frac{l}{a}, \quad h_2 = \frac{2 \Delta}{27} \frac{m}{b}, \quad h_3 = \frac{2 \Delta}{27} \frac{n}{c}

But

2Δa=ha,2Δb=hb,2Δc=hc \frac{2 \Delta}{a} = h_a, \quad \frac{2 \Delta}{b} = h_b, \quad \frac{2 \Delta}{c} = h_c

Thus we get

h1ha=l27,h2hb=m27,h3hc=n27 \frac{h_1}{h_a} = \frac{l}{27}, \quad \frac{h_2}{h_b} = \frac{m}{27}, \quad \frac{h_3}{h_c} = \frac{n}{27}

However, we also have

h1ha=[PBC]Δ,h2hb=[PCA]Δ,h3hc=[PAB]Δ \frac{h_1}{h_a} = \frac{[PBC]}{\Delta}, \quad \frac{h_2}{h_b} = \frac{[PCA]}{\Delta}, \quad \frac{h_3}{h_c} = \frac{[PAB]}{\Delta}

Adding these three relations,

h1ha+h2hb+h3hc=1 \frac{h_1}{h_a} + \frac{h_2}{h_b} + \frac{h_3}{h_c} = 1

Thus

l27+m27+n27=h1ha+h2hb+h3hc=1 \frac{l}{27} + \frac{m}{27} + \frac{n}{27} = \frac{h_1}{h_a} + \frac{h_2}{h_b} + \frac{h_3}{h_c} = 1

We conclude that l+m+n=27l + m + n = 27. Thus every good point PP determines a partition (l,m,n)(l, m, n) of 2727 such that there are l,m,nl, m, n equal segments respectively on BC,CA,ABBC, CA, AB.

Conversely, take any partition (l,m,n)(l, m, n) of 2727. Divide BC,CA,ABBC, CA, AB respectively into l,m,nl, m, n equal parts. Define

h1=2lΔ27a,h2=2mΔ27b h_1 = \frac{2 l \Delta}{27 a}, \quad h_2 = \frac{2 m \Delta}{27 b}

Draw a line parallel to BCBC at a distance h1h_1 from BCBC; draw another line parallel to CACA at a distance h2h_2 from CACA. Both lines are drawn such that they intersect at a point PP inside the triangle ABCABC. Then

[PBC]=12ah1=lΔ27,[PCA]=mΔ27 [PBC] = \frac{1}{2} a h_1 = \frac{l \Delta}{27}, \quad [PCA] = \frac{m \Delta}{27}

Hence

[PAB]=nΔ27 [PAB] = \frac{n \Delta}{27}

This shows that the distance of PP from ABAB is

h3=2nΔ27c h_3 = \frac{2 n \Delta}{27 c}

Therefore each triangle with base on CACA has area Δ27\frac{\Delta}{27}. We conclude that all the triangles which partition ABCABC have equal areas. Hence PP is a good point.

Thus the number of good points is equal to the number of positive integral solutions of the equation l+m+n=27l + m + n = 27. This is equal to

(262)=325 \binom{26}{2} = 325

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Source: MathNet, licensed CC-BY-4.0. Statement reproduced verbatim; metadata (topic, difficulty) added by this project.