How to prove this congruent proof?

[jy02354441]

If a is congruent to b mod n and a^k−1 is congruent to b^k−1 mod n, then a^k is congruent to b^k mod n.? n>0 and k>1

How to prove it by induction?

 

[Office_Shredder]

I'm assuming that's a^k-1 and not a^k-1. Do you know what the rules are for multiplication modulo n?