Number theoryDifficulty 6.1National olympiadFind the answer
Find all integers a,b,c such that a2=bc+1 and b2=ac+1
A number or a short expression. Spacing and $ signs are ignored.
Solution
To solve the given system of equations: a2=bc+1 b2=ac+1
we will proceed step-by-step.
1. Subtract the two equations: a2−b2=bc−ac Using the difference of squares on the left-hand side: (a−b)(a+b)=c(b−a) Simplifying, we get: (a−b)(a+b+c)=0
2. Consider the cases: - **Case 1: a−b=0** a=b Substitute b=a into the first equation: a2=ac+1 a2−ac−1=0 This is a quadratic equation in a. Solving for a using the quadratic formula: a=2c±c2+4 Since a must be an integer, c2+4 must be a perfect square. Let k2=c2+4, then: k2−c2=4 (k−c)(k+c)=4 The integer pairs (k−c,k+c) that satisfy this are (1,4) and (2,2). Solving these: - For (1,4): k−c=1 k+c=4 Solving these, we get: k=21+4=2.5 c=24−1=1.5 This does not yield integer solutions. - For (2,2): k−c=2 k+c=2 Solving these, we get: k=22+2=2 c=22−2=0 Substituting c=0 back into the quadratic equation: a2=1 a=±1 Thus, a=b=±1 and c=0.
- **Case 2: a+b+c=0** c=−(a+b) Substitute c=−(a+b) into the original equations: a2=b(−a−b)+1 a2=−ab−b2+1 a2+ab+b2=1 This is a quadratic Diophantine equation. We need to find integer solutions for a and b such that: a2+ab+b2=1 Testing small integer values: - For a=1: 1+b+b2=1 b2+b=0 b(b+1)=0 b=0 or b=−1 Thus, (a,b)=(1,0) or (1,−1). - For a=−1: 1−b+b2=1 b2−b=0 b(b−1)=0 b=0 or b=1 Thus, (a,b)=(−1,0) or (−1,1).
Combining these results, we have the solutions: (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)
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Source: NuminaMath-1.5,
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