.
+ - = 0.
In the left side the first two addends are the areas of the four semi-circles built on the sides of the rectangle.
The subtrahend is the area if the big circle.
Similar to that as
"the area of the square built on the hypotenuse of the right-angled triangle is equal to the sum of areas
of two squares built on its legs"
(Pythagoras),
"the area of the semi-circle built on the hypotenuse of the right-angled triangle is equal to the sum of areas
of two semi-circles built on its legs"
Answer. 0. Zero. ZERO.