Olympiad Maths Prep

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Problem 1543

National olympiad second round; IMO P1/P4
Combinatorics Difficulty 7.3 Find the answer

Four distinct points are marked in a line. For each point, the sum of the distances from said point to the other three is calculated; getting in total 4 numbers.

Decide whether these 4 numbers can be, in some order:
a) 29,29,35,3729,29,35,37
b) 28,29,35,3728,29,35,37
c) 28,34,34,3728,34,34,37

Official solution

1. Define the points and distances:
Let the four points be A,B,C,D A, B, C, D in order from left to right. Define the distances between consecutive points as follows:
AB=x,BC=y,CD=z AB = x, \quad BC = y, \quad CD = z

2. Calculate the sum of distances from each point:
- For point A A :
Sum of distances=AB+AC+AD=x+(x+y)+(x+y+z)=3x+2y+z \text{Sum of distances} = AB + AC + AD = x + (x + y) + (x + y + z) = 3x + 2y + z
- For point B B :
Sum of distances=BA+BC+BD=x+y+(y+z)=x+2y+z \text{Sum of distances} = BA + BC + BD = x + y + (y + z) = x + 2y + z
- For point C C :
Sum of distances=CA+CB+CD=(x+y)+y+z=x+2y+z \text{Sum of distances} = CA + CB + CD = (x + y) + y + z = x + 2y + z
- For point D D :
Sum of distances=DA+DB+DC=(x+y+z)+(y+z)+z=x+2y+3z \text{Sum of distances} = DA + DB + DC = (x + y + z) + (y + z) + z = x + 2y + 3z

3. Identify the necessary conditions:
- From the calculations, we see that the sums of distances from B B and C C are equal:
Sum from B=Sum from C=x+2y+z \text{Sum from } B = \text{Sum from } C = x + 2y + z
- The sums of distances from A A and D D are:
Sum from A=3x+2y+z \text{Sum from } A = 3x + 2y + z
Sum from D=x+2y+3z \text{Sum from } D = x + 2y + 3z

4. Check each option:
- **Option (a) 29,29,35,37 29, 29, 35, 37 :**
- We need two equal numbers, which are 29 29 and 29 29 . Assign these to B B and C C :
x+2y+z=29 x + 2y + z = 29
- The remaining sums are 35 35 and 37 37 . Assign these to A A and D D :
3x+2y+z=35 3x + 2y + z = 35
x+2y+3z=37 x + 2y + 3z = 37
- Solve the system of equations:
x+2y+z=29(1) x + 2y + z = 29 \quad \text{(1)}
3x+2y+z=35(2) 3x + 2y + z = 35 \quad \text{(2)}
x+2y+3z=37(3) x + 2y + 3z = 37 \quad \text{(3)}
- Subtract (1) from (2):
3x+2y+z(x+2y+z)=3529 3x + 2y + z - (x + 2y + z) = 35 - 29
2x=6    x=3 2x = 6 \implies x = 3
- Subtract (1) from (3):
x+2y+3z(x+2y+z)=3729 x + 2y + 3z - (x + 2y + z) = 37 - 29
2z=8    z=4 2z = 8 \implies z = 4
- Substitute x=3 x = 3 and z=4 z = 4 back into (1):
3+2y+4=29 3 + 2y + 4 = 29
2y=22    y=11 2y = 22 \implies y = 11
- Therefore, x=3 x = 3 , y=11 y = 11 , z=4 z = 4 satisfies all conditions. Thus, option (a) is possible.

- **Option (b) 28,29,35,37 28, 29, 35, 37 :
- We need two equal numbers, but there are no two equal numbers in this set. Thus, option (b) is not possible.

- Option (c) 28,34,34,37 28, 34, 34, 37 :**
- We need two equal numbers, which are 34 34 and 34 34 . Assign these to B B and C C :
x+2y+z=34 x + 2y + z = 34
- The remaining sums are 28 28 and 37 37 . Assign these to A A and D D :
3x+2y+z=28 3x + 2y + z = 28
x+2y+3z=37 x + 2y + 3z = 37
- Solve the system of equations:
x+2y+z=34(1) x + 2y + z = 34 \quad \text{(1)}
3x+2y+z=28(2) 3x + 2y + z = 28 \quad \text{(2)}
x+2y+3z=37(3) x + 2y + 3z = 37 \quad \text{(3)}
- Subtract (1) from (2):
3x+2y+z(x+2y+z)=2834 3x + 2y + z - (x + 2y + z) = 28 - 34
2x=6    x=3 2x = -6 \implies x = -3
- Subtract (1) from (3):
x+2y+3z(x+2y+z)=3734 x + 2y + 3z - (x + 2y + z) = 37 - 34
2z=3    z=1.5 2z = 3 \implies z = 1.5
- Substitute x=3 x = -3 and z=1.5 z = 1.5 back into (1):
3+2y+1.5=34 -3 + 2y + 1.5 = 34
2y=35.5    y=17.75 2y = 35.5 \implies y = 17.75
- Since x=3 x = -3 , y=17.75 y = 17.75 , z=1.5 z = 1.5 are not valid distances (distances cannot be negative), option (c) is not possible.

Conclusion:
Only option (a) 29,29,35,37 29, 29, 35, 37 is possible.

The final answer is 29,29,35,37 \boxed{ 29, 29, 35, 37 }

Source: NuminaMath-1.5, licensed Apache-2.0. Statement and solution reproduced as published; topic, difficulty and ordering added by this site.