The formula for finding the area of the circle works because they made it especially for it. I don’t really know how else to describe it, it simply just works.
A diagram helps.
Diagonal TO divides the rectangle into two triangles of base 10 and height 4. The area of the two of them together (the area of the rectangle) is 10*4 = 40.
The area of rectangle TYOC is 40 square units.