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Evaluate a limit by factoring to resolve a 0/0 form

Consider the function t(x)=x3−8x2−4t(x) = \dfrac{x^3 - 8}{x^2 - 4}.

Find lim⁡x→2x3−8x2−4.\lim_{x \to 2} \frac{x^3 - 8}{x^2 - 4}.

Hint

Direct substitution gives 0/00/0. Factor the numerator as a difference of cubes and the denominator as a difference of squares, then cancel the common factor before substituting.

Step-by-step solution

Substituting x=2x=2 directly gives 23−822−4=00\frac{2^3-8}{2^2-4} = \frac{0}{0}, an indeterminate form, so we cannot use direct substitution and must simplify algebraically first. Factor the numerator using the difference of cubes formula: x3−8=(x−2)(x2+2x+4)x^3 - 8 = (x-2)(x^2+2x+4). Factor the denominator using the difference of squares formula: x2−4=(x−2)(x+2)x^2 - 4 = (x-2)(x+2).

Since x≠2x \neq 2 as x→2x \to 2, the common factor (x−2)(x-2) can be canceled, giving (x−2)(x2+2x+4)(x−2)(x+2)=x2+2x+4x+2.\frac{(x-2)(x^2+2x+4)}{(x-2)(x+2)} = \frac{x^2+2x+4}{x+2}. Now apply the quotient rule for limits, since both numerator and denominator have finite limits with a nonzero denominator limit at x=2x=2: lim⁡x→2x2+2x+4x+2=22+2(2)+42+2=124=3.\lim_{x\to 2}\frac{x^2+2x+4}{x+2} = \frac{2^2+2(2)+4}{2+2} = \frac{12}{4} = 3.

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