Tuesday 7 April 2015

modular arithmetic - What is $5^{-1}$ in $mathbb Z_{11}$?

I am trying to understand what this question is asking and how to solve it. I spent some time looking around the net and it seems like there are many different ways to solve this, but I'm still left confused.




What is the multiplicative inverse of $5$ in $\mathbb Z_{11}$.

Perform a trial and error search using a calculator to obtain your answer.




I found an example here:




In $\mathbb Z_{11}$, the multiplicative inverse of $7$ is $8$ since $7 * 8 \equiv 56 \pmod {11}$.




This example is confusing to me because $1 \pmod {11} \equiv 1$. I don't see how $56$ is congruent to $1 \pmod {11}$.

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