intermediate algebra

in solving the equation (x+4)(x-4)=9 eric stated the solution would be x+4=9 =>5 or x-4=9 =>13 however at least one of these solutions fails to work when subsituted back into the original equation. why is that? solve the problem yourself and explain your reasoning

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  1. (x+4)(x-4) = 9
    x^2 -16 = 9
    x^2 = 25
    x = +5 or -5

    Eric's logic is incorrect.
    You cannot conclude from (x+4)(x-4)=9
    than x-4 = 9.
    In the other case, x + 4 = 9 does happen to be a solution, but that is a bit of a lucky guess.

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