iMac 7.1 and AutoCAD 2015

Hi,

I've an iMac 7.1 with a fresh update to OS X 10.10.3. I need to install an AutoCAD 2015 version but before to buy it, I read the system requirements and I discover that for iMac, I need to have 8.1 minimum system (11.1 or later recommended). It means that I shall not be able to install this software on my old iMac ?

Here is the Autodesk link :

http://www.autodesk.com/store/autocad-lt-for-mac/system-requirements

Thank you for your help.

PS : I'm a new apple user, so sorrry for this newbe question

Laurent