Maths Olympiad Prep

Track / Stage 6 / 319 of 400 #1319 of 1964

Problem 1319

National olympiad, first round
Number theory Difficulty 6.6 Find the answer

Let aa, bb, cc, xx, yy, and zz be positive integers such that a22x=b237y=c241z=a+b+c. \frac{a^2-2}{x} = \frac{b^2-37}{y} = \frac{c^2-41}{z} = a+b+c. Let S=a+b+c+x+y+zS=a+b+c+x+y+z. Compute the sum of all possible values of SS.

Proposed by Luke Robitaille

A number or a short expression. Fractions can be typed as 3/2, and spacing doesn't matter.

Official solution

Given the equation:
a22x=b237y=c241z=a+b+c \frac{a^2-2}{x} = \frac{b^2-37}{y} = \frac{c^2-41}{z} = a+b+c
Let M=a+b+c M = a + b + c . Since a,b,c1 a, b, c \geq 1 , we have M3 M \geq 3 .

### Lemma 1: M146 M \mid 146
Proof of Lemma 1:
Consider the quantity T=(a+b+c)(41ab+3bc+38ca) T = (a + b + c)(-41ab + 3bc + 38ca) .
Note that T0(modM) T \equiv 0 \pmod{M} .

But,
T41a2b+38a2c41b2a+3b2c+3c2b+38c2a(modM) T \equiv -41a^2b + 38a^2c - 41b^2a + 3b^2c + 3c^2b + 38c^2a \pmod{M}
Rewriting the terms, we get:
T[(41a2b+82b)+(38a2c76c)+(41b2a+1517a)+(3b2c111c)+(3c2b123b)+(38c2a1558a)]+[41a+41b+187c](modM) T \equiv [(-41a^2b + 82b) + (38a^2c - 76c) + (-41b^2a + 1517a) + (3b^2c - 111c) + (3c^2b - 123b) + (38c^2a - 1558a)] + [41a + 41b + 187c] \pmod{M}
Simplifying further:
T41a+41b+187c146c(modM) T \equiv 41a + 41b + 187c \equiv 146c \pmod{M}
Thus, M146c M \mid 146c .

Now, if gcd(M,c)=k \gcd(M, c) = k , then k41 k \mid 41 . Since gcd(146,41)=1 \gcd(146, 41) = 1 , we have M146 M \mid 146 .

This completes the proof of Lemma 1. \blacksquare

The only factors of 146 146 are 1,2,73,146 1, 2, 73, 146 . Since M3 M \geq 3 , we have M=73 M = 73 or M=146 M = 146 .

### Case 1: M=73 M = 73
We need to find a,b,c a, b, c such that:
a+b+c=73 a + b + c = 73
Given the conditions:
a22x=b237y=c241z=73 \frac{a^2-2}{x} = \frac{b^2-37}{y} = \frac{c^2-41}{z} = 73
We can solve for x,y,z x, y, z :
x=a2273,y=b23773,z=c24173 x = \frac{a^2-2}{73}, \quad y = \frac{b^2-37}{73}, \quad z = \frac{c^2-41}{73}
Checking possible values, we find:
a=32,b=16,c=25 a = 32, \quad b = 16, \quad c = 25
Thus:
x=322273=1024273=14 x = \frac{32^2-2}{73} = \frac{1024-2}{73} = 14
y=1623773=2563773=3 y = \frac{16^2-37}{73} = \frac{256-37}{73} = 3
z=2524173=6254173=8 z = \frac{25^2-41}{73} = \frac{625-41}{73} = 8
So, (a,b,c,x,y,z)=(32,16,25,14,3,8) (a, b, c, x, y, z) = (32, 16, 25, 14, 3, 8) .

Thus, S=a+b+c+x+y+z=32+16+25+14+3+8=98 S = a + b + c + x + y + z = 32 + 16 + 25 + 14 + 3 + 8 = 98 .

### Case 2: M=146 M = 146
We need to find a,b,c a, b, c such that:
a+b+c=146 a + b + c = 146
Given the conditions:
a22x=b237y=c241z=146 \frac{a^2-2}{x} = \frac{b^2-37}{y} = \frac{c^2-41}{z} = 146
We can solve for x,y,z x, y, z :
x=a22146,y=b237146,z=c241146 x = \frac{a^2-2}{146}, \quad y = \frac{b^2-37}{146}, \quad z = \frac{c^2-41}{146}
Checking possible values, we find:
a=32,b=89,c=25 a = 32, \quad b = 89, \quad c = 25
Thus:
x=3222146=10242146=7 x = \frac{32^2-2}{146} = \frac{1024-2}{146} = 7
y=89237146=792137146=54 y = \frac{89^2-37}{146} = \frac{7921-37}{146} = 54
z=25241146=62541146=4 z = \frac{25^2-41}{146} = \frac{625-41}{146} = 4
So, (a,b,c,x,y,z)=(32,89,25,7,54,4) (a, b, c, x, y, z) = (32, 89, 25, 7, 54, 4) .

Thus, S=a+b+c+x+y+z=32+89+25+7+54+4=211 S = a + b + c + x + y + z = 32 + 89 + 25 + 7 + 54 + 4 = 211 .

### Conclusion
The sum of all possible values of S S is:
98+211=309 98 + 211 = \boxed{309}

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