As Steve says, there's an adapter 'stem' that comes with the fan. It's just for your stated purpose (ie., a 'angled ceilings.)
I just put one in for my M-I-L 2 weekends ago. When you buy a new fan, they give you parts for about 4 DIFFERENT mounting scenarios, one of which includes 'angled ceilings'.
Want an unsolicited tip??! READ the directions entirely before you do any work. Then, once you see what 'section/chapter' of the 4 sets of directions you need, re-read just them and go at it. It's amazing how many screws and such come with the fan. I had so many left over that I could have re-drywalled the bedroom I put it in! (Well, maybe just the closet.) (LOL)