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MathNet 2008-part-05 — 164 problems (EN–VI) Question 120

Math Olympiad Vault · MathNet 2008-part-05 — 164 problems (EN–VI)

Question 120

Let $p$ be a prime. Given a sequence of positive integers $b_{1}$ through $b_{n}$, exactly one of which is divisible by $p$, show that when
$$
\frac{1}{b_{1}}+\frac{1}{b_{2}}+\ldots+\frac{1}{b_{n}}
$$
is written as a fraction in lowest terms, then its denominator is divisible by $p$. Use this fact to explain why no prime $p$ is ever juicy.