Math: How to call a square paper a square with half the area?

Fold the square twice along two diagonal lines to get a small isosceles triangle.

Let the side length of the original square be a, then the root of the waist length of this small isosceles triangle is 2/2/2 * a a.

Fold the paper back to its original shape.

One side of a square is folded diagonally, and this side coincides with the diagonal.

Then fold it in half along the other diagonal.

Divide this side into two sections, and the length of one section is the root number 2/2/2 * a a.

So you can get a square with half the area.