Quote:
Originally Posted by bur
Could a mod change the thread titel to something like "Help with exercise questions from Elementary Number Theory"? So I don't have to open a new threads several times. Thanks.

Done.
Related to your question, you just proved it, which is correct. What other proof do you want? (yes, the question is deliberately formulated in that tricky way, similar to "prove that if n1 is a square and n+1 is a cube, then n3, n+3, and n+5 are all prime", or the (in)famous "new mersenne conjecture", mathematicians have the right to joke too
)