some people learn it in Extension 1 because as pointed out elsewhere in this thread it is in a chapter as an "extension" topic in the cambridge prelim 3u book. It is technically within mx2 but by that stage you should have the mathematical knowledge to understand it so you can learn it now but it is unlikely to be examinable in a 3u exam (although if your school makes it's own exams and you went through this topic at school it is best to confirm with the teacher if it will be examined)
