If the radius of the circle is , what is the length of the curved line in the interior of the circle that separates the white region from the dark region?

Question

If the radius of the circle is r , what is the length of the curved line in the interior of the circle that separates the white region from the dark region?

Answer 1

Draw a diameter horizontally, The curved line consists of 2 semicircles, each with diameter r.

So, the curved line has length equal to the circumference of a circle with diameter r.

Thus, the length is 2πr.