I was working on an engineering problem where I had to find the value of a component. I used Maxima (a computer algebra system) to get my answer, except that I got two answers instead of one. Only one of the answers satisfied my initial conditions. So my question is "when do you need to check your answers"? I seem to recall some stuff about symmetrical functions or always positive or something else. Can anyone help me out or point me to some reading? If anyone wants to know the exact equations I was using, they are here. L1*L2 - M^2 ------------------- = Leq L1+L2 - 2*M where L1 = L2 = 0.2 and Leq = 0.14 ---->Solve for M The answer I got was M = 0.2 and M = 0.08 If you plug in M = 0.2 you get 0. Can anyone explain where this false answer came from?