Fermat's Last Theorem (FLT), (1637), states that if n is an integer greater than 2, then it is impossible to find three natural numbers x, y and z where such equality is met being (x,y)>0 in xn + yn = zn. This paper shows the methodology to prove Fermat's Last Theorem using Reduction ad absurdum, the Pythagorean Theorem and the property of similar triangles, known in the 17TH century, when Fermat enunciated the theorem.