If you liked this problem, here is an NRICH task which challenges you to use similar mathematical ideas.
The diagram shows two identical large circles and two identical smaller circles whose centres are at the corners of a square. The two large circles are touching, and they each touch the two smaller circles. The radius of the small circles is $1cm$. What is the radius of the large circles in centimetres?
If you liked this problem, here is an NRICH task which challenges you to use similar mathematical ideas.