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Here is the multiplication table for$\mod 7$: https://gyazo.com/82fb45bd89f61df3b44f00f67efc63c1

How do I read this to get the multiplicative inverse of something, for example like:

$5\mod 7$

I know the answer is $3$ by using a different method, but how do I get it using this table?

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  • $\begingroup$ $a$ has an inverse $b$ modulo $7$ if $ab \equiv 1 \pmod{7}$. So look for the row column entry that corresponds to $1$. $\endgroup$ – Anurag A Jul 2 '17 at 18:17
  • $\begingroup$ Find the number from the table which multiplied by $5$ gives the identity, which here is $1$. You can see the answer is $3$. $\endgroup$ – Sahiba Arora Jul 2 '17 at 18:17
  • $\begingroup$ I understand now, thanks a lot ! $\endgroup$ – BurstFlame Jul 2 '17 at 18:23
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By definition, the multiplicative inverse of an element $x$ is the element $y$ that, when the two are multiplied, returns the identity.

To read a multiplication table (or generally a Cayley table to find any operative inverse) proceed along the row for your chosen element $x$ until you reach the identity. Proceed up that column and its label element will be the inverse $y$.

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