Exercise
Let K be a field.
Suppose that f(X) is an irreducible polynomial of degree at least 2 in K[X].
Then L = K[X]/(f(X)) is a field.
What can you say about the polynomial
f(Y) in L[Y]?
It is irreducible.
It has no linear factors.
It has a linear factor.
No,
Y - X is a linear factor by Part 1 of the lemma.
No,
Y - X is a linear factor by Part 1 of the lemma.
Indeed,
Y - X is a linear factor by Part 1 of the lemma.