Came upon this question in one of the specimen papers. Assume it is a theorem of some sort, but looked in the specification and couldn't find one relating to it. Anyone know what theorem it is? It's Q19 Sorry - am using the wrong browser so can't insert hyperlinks, apparently. http://store.aqa.org.uk/resourceZone/pdf/maths/AQA-9365-2H-W-SQP.PDF The mark scheme starts off with: 20 x BX = 15 x 8, then continues to solve for BX and then finish to find the diameter. Thanks.