Question

let (a,57)=1 then a^18 congruent to 1 in modulo 57.

Answer 1

nope

Answer 2

Let think of the smallest case (2,3) =1
that means 2,3 are relative prime, but 2 is NOT congruent to 1 mod 3
That is \(2\cancel\equiv 1(mod3)\)

Answer 3

now see question after editing :D