Let us call the side length of the square a, then the area of one white triangle is
There are two of these white triangles, and therefore, the area encompassed by them is
The area A of the yellow region is equal to the area of the square minus the area of the white triangles.
The area of the square is a^2; therefore,
Meaning, the area of the yellow region is 1/2 the area of the square.