Pythagorean theorem is frequently taught to high school students. The sketch provided demonstrates the fundamentals of this theorem. Notice that the area of the green square added to the area of the purple square equals the area of the blue square. Drag a point on the triangle to see that for all right triangles, the length of the two legs of the right angle squared add together to equal the length of the hypotenuse squared.
For Fermat's theorem, let's explore what happens when a^n + b^n = c^n, where n = 3.