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Evaluating a 0/0 limit using L'Hopital's Rule

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

Evaluate the limit lim⁡x→2x3−8x2−4.\lim_{x \to 2} \frac{x^3 - 8}{x^2 - 4}. First verify that direct substitution produces an indeterminate form, then apply the appropriate technique to find the value of the limit.

Hint

Plug in x=2x=2 to confirm you get 0/00/0. Since this is indeterminate, differentiate the numerator and denominator separately (not with the quotient rule), then substitute again.

Step-by-step solution

Substituting x=2x=2 gives 23−822−4=00\frac{2^3-8}{2^2-4} = \frac{0}{0}, which is an indeterminate form, so direct substitution fails and L'Hopital's Rule applies. Differentiate the numerator and denominator separately: the derivative of x3−8x^3-8 is 3x23x^2, and the derivative of x2−4x^2-4 is 2x2x.

This gives the new limit lim⁡x→23x22x\lim_{x\to 2} \frac{3x^2}{2x}. Substituting x=2x=2 now gives 3(4)2(2)=124=3\frac{3(4)}{2(2)} = \frac{12}{4} = 3, which is no longer indeterminate, so the original limit equals 33.

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