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Domain and vertical asymptotes

Consider the function f(x)=x2−4x3−x.f(x) = \dfrac{x^2 - 4}{x^3 - x}. This function will be analyzed to build a complete curve sketch.

Factor the numerator and denominator of f(x)=x2−4x3−xf(x) = \dfrac{x^2-4}{x^3-x} completely. Using this factorization, state the domain of ff and identify all vertical asymptotes of ff.

Hint

Factor x3−xx^3 - x as x(x−1)(x+1)x(x-1)(x+1) and x2−4x^2-4 as (x−2)(x+2)(x-2)(x+2). A vertical asymptote occurs where the denominator is zero but the numerator is not zero at that same point (after cancellation, if any).

Step-by-step solution

Factor: numerator x2−4=(x−2)(x+2)x^2-4 = (x-2)(x+2), denominator x3−x=x(x−1)(x+1)x^3-x = x(x-1)(x+1). So f(x)=(x−2)(x+2)x(x−1)(x+1)f(x) = \dfrac{(x-2)(x+2)}{x(x-1)(x+1)}. There are no common factors between numerator and denominator, so no holes occur.

The denominator is zero at x=0,1,−1x=0,1,-1, and the numerator is nonzero at each of these points, so all three are vertical asymptotes. The domain is all real numbers except x=0,1,−1x=0,1,-1, i.e. (−∞,−1)∪(−1,0)∪(0,1)∪(1,∞)(-\infty,-1)\cup(-1,0)\cup(0,1)\cup(1,\infty).

Intercepts

Find all xx-intercepts and the yy-intercept of f(x)=x2−4x3−xf(x) = \dfrac{x^2-4}{x^3-x}, or explain why one does not exist.

Hint

For x-intercepts, set the numerator equal to zero (checking these points are in the domain). For the y-intercept, evaluate f(0)f(0) if it exists.

Step-by-step solution

Set the numerator equal to zero: x2−4=0x^2 - 4 = 0 gives x=2x = 2 and x=−2x = -2. Both values are in the domain (they don't make the denominator zero), so the xx-intercepts are (2,0)(2,0) and (−2,0)(-2,0).

For the yy-intercept, we need f(0)f(0), but x=0x=0 is not in the domain since the denominator vanishes there. Therefore there is no yy-intercept.

Horizontal asymptote

Determine the horizontal asymptote (if any) of f(x)=x2−4x3−xf(x) = \dfrac{x^2-4}{x^3-x} by evaluating lim⁡x→∞f(x)\lim_{x\to\infty} f(x).

Hint

Compare the degree of the numerator (2) to the degree of the denominator (3), or divide numerator and denominator by the highest power of xx in the denominator.

Step-by-step solution

Divide numerator and denominator by x3x^3: f(x)=1x−4x31−1x2f(x) = \dfrac{\frac{1}{x} - \frac{4}{x^3}}{1 - \frac{1}{x^2}}. As x→∞x\to\infty, every term with xx in the denominator goes to 0, leaving 01=0\dfrac{0}{1} = 0.

Since the degree of the numerator (2) is less than the degree of the denominator (3), the limit as x→±∞x\to \pm\infty is 0 in both directions, so y=0y=0 is a horizontal asymptote.

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