The basis for Fermat's Last Theorem was Pythagoras's theorem. The latter showed that in any right angled triangle, the lengths of the sides satisfies
a^2 + b^2 = c^2.
In particular, that there are integer solutions to the equation: such as {3, 4, 5} or {5, 12, 13}.
Fermat's theorem proved that there were no non-trivial solutions for
a^n + b^n = c^n for any positive integers a, b, c and n where n > 2
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Fermat's last theorem states that the equation xn + yn = zn has no integer solutions for x, y and z when the integer n is greater than 2. When n=2, we obtain the Pythagoras theorem.
Although the Pythagorean theorem (sums of square of a right angled triangle) is called a theorem it has many mathematical proofs (including the recent proof of Fermats last theorem which tangentially also prooves Pythagorean theorem). In fact Pythagorean theorem is an 'axiom', a kind of 'super law'. It doesn't matter if anyone does oppose it, it is one of the few fundamental truths of the universe.
QED, Fermat's Last Theorem.
But it was. That is why we know about it. If you mean why the PROOF was not written- Fermat wrote that he had found a wonderful proof for the theorem, but unfortunately the margin was too small to contain it. This is why the theorem became so famous- being understandable by even a schoolchild, but at the same time so hard to prove that even the best mathematicians had to surrender, with a simple proof seemingly being existent that just nobody except Fermat could find. The theorem has since been proven but the proof uses math tools that are very advanced and were not available in Fermat's life-time.
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