How can i Prove that the gray area is the same as white area?
Carter and Wagon's proof without words:
(Image author: Christian Lawson-Perfect, source)
This theorem is known as the Pizza theorem (see https://en.wikipedia.org/wiki/Pizza_theorem for further reading). There exist algebraic proofs, but the most elegant proof for the 8 segment case is below:
In general, alternating areas are equal iff the number of segments is a multiple of 4 at least 8.