The crescent theorem means that if the two right-angled sides of a right-angled triangle are made into two semicircles outward and the hypotenuse is made into a semicircle inward, the sum of the areas of the two crescent shapes surrounded by the three semicircles is equal to the area of the right-angled triangle.

This is a beautiful theorem discovered by Hippocrates, an ancient Greek mathematician, which is widely used in plane geometry.