# Thread: proof prob

1. ## proof prob

y = x^2 + x + 11 The value of y is a prime number when x = 0, 1, 2 and 3

The following statement is not true:
y = x^2 + x + 11 is always a prime number when x is an integer’

Show that the statement is not true.

Thanks,

2. Originally Posted by GAdams
The following statement is not true:
y = x^2 + x + 11 is always a prime number when x is an integer’

Show that the statement is not true.
Just give a counterexample: $x=10$ will work.

3. Originally Posted by Reckoner
Just give a counterexample: $x=10$ will work.
I was (mistakenly) reading it as a proof problem. But yes, that works. Thanks.

4. Originally Posted by GAdams
I was (mistakenly) reading it as a proof problem. But yes, that works. Thanks.
Well, really, it is a proof. But since we are only trying to prove the existence of a single value (specifically, you are trying to prove that there is at least one value for which the expression is not prime) the proof consists merely of finding such a value.