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One-sided and two-sided limits of a simplifiable rational function

Consider the function f(x)=x3−8x2−4f(x) = \dfrac{x^3 - 8}{x^2 - 4}. Note that f(x)f(x) is undefined at x=2x = 2 since both the numerator and denominator equal zero there.

The table below shows values of f(x)=x3−8x2−4f(x) = \dfrac{x^3-8}{x^2-4} for xx near 22, approaching from both sides.

xx1.91.991.99922.0012.012.1
f(x)f(x)2.91752.99172.9992undefined3.00083.00753.0758

Using this information (and/or algebraic simplification of f(x)f(x)), find lim⁡x→2−f(x)\displaystyle\lim_{x\to 2^-} f(x), lim⁡x→2+f(x)\displaystyle\lim_{x\to 2^+} f(x), and lim⁡x→2f(x)\displaystyle\lim_{x\to 2} f(x), if each exists. State the value of f(2)f(2) itself, if it exists, and explain how it relates to the limit.

Hint

Factor the numerator as a difference of cubes and the denominator as a difference of squares, then cancel the common factor (x−2)(x-2) before evaluating the simplified expression at x=2x=2. Remember that the limit only depends on values of xx near 2, not on x=2x=2 itself.

Step-by-step solution

Factor both numerator and denominator: x3−8=(x−2)(x2+2x+4)x^3 - 8 = (x-2)(x^2+2x+4) and x2−4=(x−2)(x+2)x^2 - 4 = (x-2)(x+2). For x≠2x \neq 2, the common factor (x−2)(x-2) cancels, giving f(x)=x2+2x+4x+2f(x) = \dfrac{x^2+2x+4}{x+2}. This simplified form agrees with f(x)f(x) everywhere except at x=2x=2, which is exactly what matters for evaluating limits since limits ignore the point x=ax=a itself.

Substituting x=2x=2 into the simplified expression gives 4+4+44=124=3\dfrac{4+4+4}{4} = \dfrac{12}{4} = 3. Since this simplified function is continuous at x=2x=2, both the left-hand and right-hand limits equal 3, matching the table's trend (values approach 3 from both sides). Therefore lim⁡x→2−f(x)=3\lim_{x\to2^-}f(x) = 3, lim⁡x→2+f(x)=3\lim_{x\to2^+}f(x) = 3, and since these one-sided limits agree, lim⁡x→2f(x)=3\lim_{x\to2}f(x) = 3. However, f(2)f(2) itself is undefined (0/0), illustrating that the limit exists even though the function has no value at that point.

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