Question

Find the additive inverse and the multiplicative inverse, if it exists, of the given number. 5 in modulo 8 arithmetic

Answer 1

The additive inverse of 5 modulo 8 is the number a such that

5 + a ≡ 0 (mod 8)

Then

a ≡ -5 ≡ -5 + 8 ≡ 3 (mod 8)

The multiplicative inverse is m such that

5m ≡ 1 (mod 8)

Use the Euclidean algorithm:

8 = 1•5 + 3

5 = 1•3 + 2

3 = 1•2 + 1

Then

1 = 3 - 1•2

1 = 3 - 1•(5 - 1•3) = 2•3 - 1•5

1 = 2•(8 - 1•5) - 1•5 = 2•8 - 3•5

and so

1 ≡ 2•8 - 3•5 ≡ (-3)•5 (mod 8)

which means the inverse of 5 is

-3 ≡ 8 - 3 ≡ 5 (mod 8)