Greatest Common Divisor in a Diophantine Equation

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Problem:

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Question:

How did the author derive $$gcd(p-1,d)=1$$? how it is true?

Reference:

  1. Page 33 of An Introduction to Diophantine Equations by Titu Andreescu, Dorin Andrica, Ion Cucurezeanu
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$p$ is the least prime factor of $d$. So $p-1$ contains no prime factor of $d$... (if it did $p$ wouldn't be the least such). Since $p-1$ and $d$ have no prime factor in common, they are coprime.