So you think you're good at trigonometry? Find the diameter of the circle.

The solution

If you can't figure it out, I'll tell you the solution, but there's a catch: you first have to purchase a book from (Perhaps unashamedly sneaky of me but this helps support and you get to read a good book.)

Just click on any Amazon banner, like the one below, (or any book reference link from an article) from any page on and buy something from You don't necessarily have to buy a book; you can purchase a CD, video, or whatever you want. After your order, just come back to and go to the email page and tell me what you purchased and ask for the solution to the puzzle. I'll then send you to a solution. Don't worry, I'll know if you purchased that item because Amazon tells me what got ordered (even though they can't tell me who ordered it).

If you can't figure out what to buy, I highly recommend one of the books from the Book Reviews.

If you click on the above banner (or any other banner on the site), and purchase something, will get a free donation from you.