Theorem: For all integers k, if k > 0 then k2 + 2k + 1 is composite.
“Proof: Suppose k is any integer such that k > 0. If
k2 + 2k + 1 is composite, then k2 + 2k + 1 = rs for some
integers r and s such that
1 < r < (k2 + 2k + 1)
and 1 < s < (k2 + 2k + 1).
Since k2 + 2k + 1 = rs
and both r and s are strictly between 1 and k2 + 2k + 1,then k2 + 2k + 1 is not prime. Hence k2 + 2k + 1 is composite
as was to be shown.”
This incorrect proof exhibits circular reasoning. The word
since in the third sentence is completely unjustified. The
second sentence tells only what happens if k2 + 2k + 1 is
composite. But at that point in the proof, it has not been
established that k2 + 2k + 1 is composite. In fact, that is
exactly what is to be proved.