Why Describe the Extensive State Using Intensive and Extensive Variables?

[Bert]

Hello,
In my book:
[PLAIN]http://img84.imageshack.us/img84/6264/variabelwk1.jpg[/PLAIN]

But why do the tell me that the need do discribe the extensieve state of a system by using intensive and at least one extensive variable?

Thanks a lot.

 

[Zeppos10]

Hello Bert: you are probably retired from PF by now but the issue might be interesting for others too:
we take a system consisting of 1 kind of species present (we take helium). internal energy U is an extensive quantity so we need 1+2=3 variables to know it. But U=n(3/2)RT, so here two variables seem to be sufficient, with n extensive and T intensive. In general thermodynamics takes U=U(S,V) which also has only 2 varibles, but for G=G(p,T) 2 would not be enough because both p and T are intensive. ??
Sorry for the mess.