Symbolic solving with defined parameters

Hi,

Sorry if this is a trivial question, but I am only starting to use symbolics, and I haven't found how to do what I want.

If I manipulate equations that are fully symbolic, the results of manipulations are shown in a symbolic way. But if there is one of the symbols that is defined, it is automatically evaluated and replaced by its value in the results. Is it possible to do that ?

Here is an example, calculating the power transfered by sinewave voltage and current with a phase offset. In the first case, the peak voltage is undefined, and I get the expected result : P(phase) = Vpk * Ipk * cos(phase) / 2

In the second case, I define Vpk=100, and the result is P(phase) = 50 * Ipk * cos(phase).

Is there a way to modify the second case, in order to get the same result as the first ?

Thank you very much.

Adrien