In math, dividing by zero is undefined. Hence, anytime you have an x in the bottom of a fraction, there will be some value(s) that would make that denominator zero. These values are called excluded values, or values that are excluded from the domain (the set of possible inputs.) To find the excluded values, factor the bottom and set each factor equal to zero to solve for x. The resulting x-values are excluded from the domain, because if x were one of these values, you'd be dividing by zero. Even if you cross out a factor on top and bottom, that factor will still tell you an excluded value for the original expression.
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