Problem: Let a, b and c be positive integers such that b2013+ca2013+b is a rational number. Show that a+b+ca2+b2+c2 and a+b+ca3−2b3+c3 are both integers.
Solution
Solution: By rationalizing the denominator, b2013+ca2013+b=2013b2−c22013ab−bc+2013(b2−ac). Since this is rational, then b2−ac=0. Consequently, a2+b2+c2=a2+ac+c2=(a+c)2−ac=(a+c)2−b2=(a−b+c)(a+b+c) and a3−2b3+c3=a3+b3+c3−3b3=a3+b3+c3−3abc=(a+b+c)(a2+b2+c2−ab−bc−ca) Therefore, a+b+ca2+b2+c2=a−b+c and a+b+ca3−2b3+c3=a2+b2+c2−ab−bc−ca are integers.
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Source: MathNet,
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