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> If $a,p\in\mathbb{N}$ with $p$ prime, have to show that if $a²\equiv1\pmod p $, then $a\equiv1\pmod p$ or $a\equiv p-1\pmod p$
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I'm studying congruence, and I have no idea where to start this demonstration, if anyone can do it, preferably detailed, or go giving ways, and I doing, I will thank enough.