Ok, so here is the deal: Ever since my 8th grade year (when I took Geometry), I've used my trusty Ti-84 Plus Silver Edition graphing calculator. Now that I've finished Pre-Calculus, and am going into AP Calculus and AP Physics, I'm thinking it could be time to upgrade. I've been looking a lot at the Ti-89 Titanium Edition, which has some nice 3D graphing and algebra functions. My sister is going into Geometry next year, so I would give her my old 84. Only problem with this is that the 89 doesn't have the same commonality with the Ti-83 that the 84 does, so I would have to translate any instructions my math teacher gives us to work on the 89. There are other people I know who use 89s, though, and they have no issues with it. Anyone here know enough about graphing calculators to tell me if its a good idea to get the 89, or should I just keep my 84 and get my sister either an 83 or an 84? Thanks!