First, sketch the square formed by the centers of the discs. Then note that the straight portions of the belt are equal in length to the corresponding sides of the square. So the sum of the straight portions of the belt is equal to the perimeter of the square.
Also note that there are 4 curved portions of the belt and each of them are 1/4 of the circumference of a disc....
-Dan
I'm not sure that I get what you mean, sorry.
Could you try and explain it again, please? I think I get the bit about the corners being a 1/4 of the circumference of a circle.
When you say sketch the square, do you mean mark the center of each circle and join up?
EDIT: I get it now. Thank you very much! It's very appreciated!