Question: Low level substitution of multiplication of symbols


It might be a very basic question, but I am having trouble with the substitution of multiplication of symbols.  I have a equation in the format of     N*x*y-x*y+f(x,y)=0, and I need to substitute xy=1 in this equation without interferring in f(x,y).

When I am using subs commnad in maple, it is not recognizing xy nad giving me the same equation.

I really appreciate help in this matter.

Thanks in advance



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