The following is a homework question:
Let P(x) be a polynomial with integer coefficients and P(x1)=P(x2)=P(x3)=P(x4)=P(x5)=P(x6)=P(x7)=7 where xi are distinct integers. Determine if P(x) has integer zeros.
I've never done questions like this before. I started with this:
If deg(P)=7,
P(x)=α(x−x1)(x−x2)(x−x3)(x−x4)(x−x5)(x−x6)(x−x7)+7
where α is some integer.
However, the question doesn't state that the polynomial must be of the seventh degree. Even then, I don't see how I can determine if P(x) has integer zeros without knowing all the xi.
Can someone please help me? Thanks.
Edit:
Is this a valid solution?
P(x)=Q(x)(x−x1)(x−x2)(x−x3)(x−x4)(x−x5)(x−x6)(x−x7)+7
If P(n)=0,
Q(n)(n−x1)(n−x2)(n−x3)(n−x4)(n−x5)(n−x6)(n−x7)=−7
where Q(x) is a polynomial of integer coefficients (therefore Q(n) is an integer)
And since all the terms on the LHS are integers and the xi are distinct integers, it follows that some of the factors on the LHS ≠ {±1,±7}. And 7 is a prime number, therefore n cannot be a zero.
Answer
Hint:
P(x)=Q(x)(x−x1)(x−x2)(x−x3)(x−x4)(x−x5)(x−x6)(x−x7)+7
Idea : You can't get −7 by multiplying 7 distinct integers..
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