'Just do its best'...
Hi! I hope this isn't a too-old thread! But anyway i wanted reply 'cause i had the same problem witch constricted me to switch between windows and mac os for several months until some weeks ago.
Microsoft Mathematics it's one of the Microsoft's best products, and it's free; very rare thing, as someone said before me!
You should be able to install it also on mac os through Crossover; but I didn't because it needs .Net Framework 3.5 and I couldn't install it. So i resolved in this way:
* Use the mac os-integrated utility Grapher to plot graphics
* Use Calcline for calculations as expression or verifying equations' results
* I use a version of Derive too, installed through crossover, for special exigencies (have the result of an equation, for example).
The last two are not free, so you've to pay them.

Anyway i thing it's a good solution. It depends also from what level of mathematic you use to study (for example, if you're in high school, or university, or what-so-ever...).
Just a final note: any of these programs shows you the steps to resolve an equation! Actually just MS Mathematics does it!
Hoping i said useful thing, bye
*sorry for my english, i'm not motherlanguage!*