and you're wrong. Thanks.
3u maths does not stuff up your 2u mark... it, in fact, makes it better. Take my ext 1 maths class, for example. We have 15 people, 11 of them doing the 2u and ext 1 course (this is year 12 though)...the placings at the end of year 12: 1st-3rd and 5th-equal 11th in 2unit....... a 2 unit student came 4th.... so eliminating the 2u student who came 4th, the whole ext 1 class got the best ranks for the 2unit course.
look, if you think you're capable of doing ext 1 in year 11, then choose it. If you go well in it, then continue it for year 12. .... honestly, it isn't that much more work. Also, it will make your 2u marks better, because you learn better ways of doing a particular question and may apply 3u stuff to it to answer it....you still get the marks.