Find the area of the shaded region

Question

http://polo.k12.mo.us/StaffPages/hholt_web/PDFLsns/Algebra3/Lsn01.pdf

Hello! Here is a link of where my problem is. If you scroll down to the second to last "slide," theres a circle with two smaller circles on there. My question is not how to do the problem. my question is how to find the area of the big circle. Now, obviously you use the area of a circle equation, but where do they get the numbers for the ratio? How does that make sense? Like the 18+18+8+8/2. These numbers I do not get. Other than That, I don't need anything else! Thanks!

Answer 1

The diameter of circle A is 36 (18 + 18).

The diameter of circle B is 16 (8 + 8)

The diameter of the large circle is 52, (36 + 16). And its radius is 26 (18 + 18 + 8 + 8) / 2.