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Number theory Difficulty 6.1 National olympiad Find the answer

Find all integers a,b,ca,b,c such that a2=bc+1a^2 = bc + 1 and b2=ac+1b^2 = ac + 1

A number or a short expression. Spacing and $ signs are ignored.

Solution

To solve the given system of equations:
a2=bc+1 a^2 = bc + 1
b2=ac+1 b^2 = ac + 1

we will proceed step-by-step.

1. Subtract the two equations:
a2b2=bcac a^2 - b^2 = bc - ac
Using the difference of squares on the left-hand side:
(ab)(a+b)=c(ba) (a - b)(a + b) = c(b - a)
Simplifying, we get:
(ab)(a+b+c)=0 (a - b)(a + b + c) = 0

2. Consider the cases:
- **Case 1: ab=0 a - b = 0 **
a=b a = b
Substitute b=a b = a into the first equation:
a2=ac+1 a^2 = ac + 1
a2ac1=0 a^2 - ac - 1 = 0
This is a quadratic equation in a a . Solving for a a using the quadratic formula:
a=c±c2+42 a = \frac{c \pm \sqrt{c^2 + 4}}{2}
Since a a must be an integer, c2+4 c^2 + 4 must be a perfect square. Let k2=c2+4 k^2 = c^2 + 4 , then:
k2c2=4 k^2 - c^2 = 4
(kc)(k+c)=4 (k - c)(k + c) = 4
The integer pairs (kc,k+c)(k - c, k + c) that satisfy this are (1,4)(1, 4) and (2,2)(2, 2). Solving these:
- For (1,4)(1, 4):
kc=1 k - c = 1
k+c=4 k + c = 4
Solving these, we get:
k=1+42=2.5 k = \frac{1 + 4}{2} = 2.5
c=412=1.5 c = \frac{4 - 1}{2} = 1.5
This does not yield integer solutions.
- For (2,2)(2, 2):
kc=2 k - c = 2
k+c=2 k + c = 2
Solving these, we get:
k=2+22=2 k = \frac{2 + 2}{2} = 2
c=222=0 c = \frac{2 - 2}{2} = 0
Substituting c=0 c = 0 back into the quadratic equation:
a2=1 a^2 = 1
a=±1 a = \pm 1
Thus, a=b=±1 a = b = \pm 1 and c=0 c = 0 .

- **Case 2: a+b+c=0 a + b + c = 0 **
c=(a+b) c = -(a + b)
Substitute c=(a+b) c = -(a + b) into the original equations:
a2=b(ab)+1 a^2 = b(-a - b) + 1
a2=abb2+1 a^2 = -ab - b^2 + 1
a2+ab+b2=1 a^2 + ab + b^2 = 1
This is a quadratic Diophantine equation. We need to find integer solutions for a a and b b such that:
a2+ab+b2=1 a^2 + ab + b^2 = 1
Testing small integer values:
- For a=1 a = 1 :
1+b+b2=1 1 + b + b^2 = 1
b2+b=0 b^2 + b = 0
b(b+1)=0 b(b + 1) = 0
b=0 or b=1 b = 0 \text{ or } b = -1
Thus, (a,b)=(1,0) (a, b) = (1, 0) or (1,1) (1, -1) .
- For a=1 a = -1 :
1b+b2=1 1 - b + b^2 = 1
b2b=0 b^2 - b = 0
b(b1)=0 b(b - 1) = 0
b=0 or b=1 b = 0 \text{ or } b = 1
Thus, (a,b)=(1,0) (a, b) = (-1, 0) or (1,1)(-1, 1).

Combining these results, we have the solutions:
(a,b,c)=(1,0,1),(1,1,0),(1,0,1),(1,1,0) (a, b, c) = (1, 0, -1), (1, -1, 0), (-1, 0, 1), (-1, 1, 0)

The final answer is (a,b,c)=(1,0,1),(1,1,0),(1,0,1),(1,1,0) \boxed{ (a, b, c) = (1, 0, -1), (1, -1, 0), (-1, 0, 1), (-1, 1, 0) }

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