Maths Olympiad Prep

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Geometry Difficulty 6.1 National Olympiad Prove it Bulgaria

Problem:
A plane intersects a tetrahedron ABCDABCD and divides the medians of the triangles DABDAB, DBCDBC and DCADCA through DD in ratios 1:21:2, 1:31:3 and 1:41:4 from DD, respectively. Find the ratio of the volumes of the two parts of the tetrahedron cut by the plane.
Oleg Mushkarov

Solution

Solution:
Let the plane meet the edges DADA, DBDB and DCDC at points PP, QQ and RR, respectively. Set
DPDA=x,DQDB=y, and DRDC=z \frac{DP}{DA} = x, \quad \frac{DQ}{DB} = y, \text{ and } \frac{DR}{DC} = z
Let MM be the midpoint of ABAB and L=DMPQL = DM \cap PQ. It follows from the condition of the
Figure 1
problem that DLDM=13\frac{DL}{DM} = \frac{1}{3}. Therefore
SDLPSDAM=DPDLDADM=x3;SDLQSDMB=DLDQDMDB=y3 \frac{S_{DLP}}{S_{DAM}} = \frac{DP \cdot DL}{DA \cdot DM} = \frac{x}{3} ; \quad \frac{S_{DLQ}}{S_{DMB}} = \frac{DL \cdot DQ}{DM \cdot DB} = \frac{y}{3}
Since SDAM=SDMB=12SDABS_{DAM} = S_{DMB} = \frac{1}{2} S_{DAB} we conclude that
2xy=2DPDQDADB=SDPQ12SDAB=SDPLSDAM+SDLQSDMB=x+y3 2xy = 2 \frac{DP \cdot DQ}{DA \cdot DB} = \frac{S_{DPQ}}{\frac{1}{2} S_{DAB}} = \frac{S_{DPL}}{S_{DAM}} + \frac{S_{DLQ}}{S_{DMB}} = \frac{x + y}{3}
i.e. 1x+1y=6\frac{1}{x} + \frac{1}{y} = 6. Analogously 1y+1z=8\frac{1}{y} + \frac{1}{z} = 8 and 1z+1x=10\frac{1}{z} + \frac{1}{x} = 10. Solving this system we obtain x=14x = \frac{1}{4}, y=12y = \frac{1}{2} and z=16z = \frac{1}{6}. Thus
VDPQRVDABC=DPDQDRDADBDC=xyz=148 \frac{V_{DPQR}}{V_{DABC}} = \frac{DP \cdot DQ \cdot DR}{DA \cdot DB \cdot DC} = xyz = \frac{1}{48}
and therefore the desired ratio equals 1:471:47.

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