Maths Olympiad Prep

Library / /84 of 136

, 1997

Geometry Difficulty 8.0 Shortlist Prove it Hong Kong

ABCABC is a triangle with integral sides. MM is the midpoint of BCBC. The in-circle with centre II touches ABAB and ACAC at EE and FF respectively and DD is the projection of MM on EFEF. Suppose that ADMIADMI is a parallelogram and AB+BC+CA=65AB + BC + CA = 65. Find AB×BC×CAAB \times BC \times CA.

Solution

The product is 93609360.
Let aa, bb, cc, rr, ss be the lengths of BCBC, CACA, ABAB, the inradius and the semiperimeter of ABC\triangle ABC respectively. Let NN be the midpoint of ACAC, and let PP be the intersection point of EFEF and MNMN. It is well-known that BPC=90\angle BPC = 90^\circ. Therefore, we have MP=a2MP = \frac{a}{2}. (Alternatively, one can prove this by noting MN=c2MN = \frac{c}{2} and PN=NF=ca2PN = NF = \frac{c-a}{2}.)

Figure 1

Note that NMABNM \parallel AB by the midpoint theorem. Since ADMIADMI is a parallelogram, we have
AI=DM=PMsinDPM=a2sinFEA=a2cosA2. AI = DM = PM \sin \angle DPM = \frac{a}{2} \sin \angle FEA = \frac{a}{2} \cos \frac{A}{2}.
On the other hand, we have
AI=rsinA2=bcsinA2ssinA2=bcscosA2. AI = \frac{r}{\sin \frac{A}{2}} = \frac{bc \sin A}{2s \sin \frac{A}{2}} = \frac{bc}{s} \cos \frac{A}{2}.
Equating these, we obtain 2bc=as2bc = as. As it is given that 2s=652s = 65, this yields 4bc=65a4bc = 65a. Then the given condition becomes 4bc65+b+c=65\frac{4bc}{65} + b + c = 65. This can be factorized as
(4b+65)(4c+65)=5652=53132. (4b + 65)(4c + 65) = 5 \cdot 65^2 = 5^3 \cdot 13^2.
As 4b+654b + 65 and 4c+654c + 65 are larger than 6565, we must have 4b+65=1254b + 65 = 125 and 4c+65=1694c + 65 = 169 up to permutation. This gives (a,b,c)=(24,15,26)(a, b, c) = (24, 15, 26) or (24,26,15)(24, 26, 15). Therefore, abc=24×15×26=9360abc = 24 \times 15 \times 26 = 9360.

Want a route through all this instead of an archive? The track puts 2,000 problems in a working order, from AMC 10 level to the IMO shortlist.

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