I always see people when doing partial fraction decomposition just plug in arbitrary values of x to cancel out some constant terms like A,B,C in order to solve it. I just want to know how this works. I've heard that since A,B, and C are constants it has to hold true for any values of x(like a function). How does this work? I get confused when I see people changing the value of x. I though x was established and you can't change it. Is this true?