Extract Real parts from an equation??

"variable is undefined" is an error mesaage from the numeric engine. All variables have to be defined, otherwise a numerical evaluation is not possible. The way to satisfay the numerics is either use the boolean "=" to make an equation (which you may solve symbolically for a distinc variable) or turn the assignment into a function, e.g. write U(R,I):=R*I instead of U:=R*I.

In your worksheet there is no equation, at least not at the and. The expression you have at the end could be separeted symbolically in real and imaginary part by using the Re(...) and Im(...) function, but Mathcad can't do that, as all of your variables could be complex numbers, too. Adding the modifyer "assume, ALL=real" or even better "assume,ALL>0" if thats appropriate would do the job, but not in case of your expression because of the square root of (x+yi). Guess you couldn't separate real and imaginary part by hand either.