Question: Evaluation time

Hello,

Hopefully, anyone can help me with the following problem:

I have two functions:  Inta=f(Pa) and ETa=f(Pna). Both can not be solved analytically, but I can plot them separately when I use >plot (evalf(Inta), Pa=0..100) and >plot(evalf(ETa), Pna=0..100). So far no problem.

Furthermore, Pna can be described as >Pna:=Pa-Inta, so in fact ETa=f(Pa). But I fail to plot ETa against Pa (or at least it takes ages..since it keeps 'evaluating'..... for days) . I tried several things like:

>plot(ETa,Pa=0..100);

>plot(evalf(ETa),Pa=0..100);

But I don't get a plot, only 'evaluating....'.

The equations for Inta and ETa are:

Inta := Pa*Int(Pm*(1-exp(-1.220/(Pm^.45-.2*Pm^.69+0.2e-1*Pm)))*exp(-Pm/(0.97e-1*Pa+8.57))/(0.97e-1*Pa+8.57), Pm = 0 .. infinity)/(0.97e-1*Pa+8.57)

ETa:= 9.285051068*Int(9.285051068*min(.5676676416*Pnm, 50.)*exp(-9.285051068*Pnm/Pna)/Pna, Pnm = 0. .. Float(infinity))

Does anybody know how I can do this?

Thanks in advance,

Miriam