how do I prove this?

[sullen mulch]

1. Ask your question and show the work you've done so far. If you've posted a screenshot of a question, specify which part you need help with.
2. Wait patiently for a helper to come along.
3. Once someone helps you, say thank you and close the thread with:

   +close
   

4. Feel free to nominate the person for helper of the week in #helper-nominations
5. Do not ping the mods, unless someone is breaking the rules. If there is a conflict amongst multiple helpers feel free to ping “Helper Mod”
6. If you're happy with the help you got here, and the server overall, you can contribute financially as well:

[keen vortex]

14. a) False. In Z/4Z we have 2.2=4=0 (mod 4)
    b) True. If ab=0 mod p, it exists k in N such as ab=pk
    Or p|ab by Gauss lemma if p dont divides a, a and p are primes to each other (because p is prime), so p divides b.
    And a=0 or b=0 mod p

Where is 18) b. ?

[candid steppe]

Figured it out! Thank you though!

[uncut magnet]

@candid steppe has given 1 rep to @keen vortex

[vernal topaz]

+close