you plug it what the lim is approaching towards to the f(x) function if the numerator is equal to positive and the denominator equal to positive than the lim is approaching positive infinity. If the numerator is approaching positive and the denominator is zero you are approaching positive infinity. If the numerator is negative and the denominator is negative, you are approaching negative infinity. If the numerator is negative and the denominator is zero you are approaching neative infinity
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This is only if there is one function in f(x)