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(1) The graph of \( f(x) = -\frac{6}{x+2} - 3 \) has a vertical asymptote at \( x = -2 \) and a horizontal asymptote at \( y = -3 \). It consists of two branches that approach these asymptotes and pass through the points \( (-4, 0) \) and \( (0, -6) \).
(2) Vertical asymptote: \( x = -2 \).
(3) Horizontal asymptote: \( y = -3 \).
(4) Domain: All real numbers except \( x = -2 \).
(5) Range: All real numbers except \( y = -3 \).
(6) The graph is symmetric with respect to the point \( (-2, -3) \), but there are no lines of symmetry.
Solución
We are given
f(x) = –6/(x + 2) – 3.
A good way to understand and sketch f(x) is to view it as a transformation of the basic reciprocal function g(u) = 1/u.
Step 1. Write f in transformation form.
Notice that
f(x) = –6/(x + 2) – 3 = [–6/(x + 2)] – 3.
If we let u = x + 2, then g(u) = 1/u is multiplied by –6 and shifted downward by 3. Thus, compared to y = 1/x, the graph:
• is horizontally shifted left by 2 (since x has changed to x + 2),
• is vertically stretched (by a factor of 6) and reflected about the horizontal axis (because of the –6 in the numerator),
• and then is shifted downward by 3.
Now we answer each part.
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(1) Sketch the graph of f
To sketch f(x) = –6/(x + 2) – 3:
• Draw the vertical asymptote (see (2) below) at x = –2.
• Draw the horizontal asymptote (see (3) below) at y = –3.
• Notice that the graph will have two separate branches.
• When x is very large (or very small), the –6/(x + 2) term becomes very small, so the graph approaches y = –3.
• Find a couple of points. For example:
– When x = 0, u = 0 + 2 = 2 so f(0) = –6/2 – 3 = –3 – 3 = –6.
– To find an x-intercept, set f(x) = 0:
–6/(x + 2) – 3 = 0 ⟹ –6/(x + 2) = 3 ⟹ x + 2 = –6/3 = –2 ⟹ x = –4.
So the graph passes through (–4, 0) and (0, –6).
• Finally, note that because of the –6/(x+2) factor the branch for x > –2 will be one “type” (falling toward the horizontal asymptote as x → ∞ and shooting off to ±∞ as x→ –2⁺), while the branch for x < –2 will be of the opposite curve.
Sketch these two branches along with the asymptotes to complete the graph.
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(2) Vertical Asymptote
The vertical asymptote occurs when the denominator is zero. Since
x + 2 = 0 ⟹ x = –2,
the vertical asymptote is
x = –2.
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(3) Horizontal Asymptote
For large |x| the term –6/(x + 2) tends to 0, so
f(x) ≈ –3.
Thus the horizontal asymptote is
y = –3.
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(4) Domain
We cannot allow a denominator of zero. Since x + 2 = 0 when x = –2, the domain is all real numbers except –2. In set‐notation,
Domain = { x ∈ ℝ : x ≠ –2 }.
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(5) Range
Since f(x) = –6/(x + 2) – 3 can never equal –3 (because –6/(x + 2) is never 0), the horizontal asymptote is actually a value that f does not attain. Hence the range is all real numbers except –3:
Range = { y ∈ ℝ : y ≠ –3 }.
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(6) Axes of Symmetry
Many transformed reciprocal functions do not have any lines (axes) of symmetry. Instead, they have point symmetry. If we rewrite f in the form
f(x) + 3 = –6/(x + 2),
and let u = x + 2, then we have g(u) = –6/u, which is an odd function because
g(–u) = –6/(–u) = 6/u = –[–6/u] = –g(u).
The graph of an odd function is symmetric with respect to the origin. In our case, because we made the substitution u = x + 2, the “origin” in u corresponds to the point where x + 2 = 0 and f(x) + 3 = 0; that is, the point
(x, y) = (–2, –3).
Thus the graph of f is symmetric with respect to the point (–2, –3) (a central symmetry), but it is not symmetric about any line (vertical or horizontal).
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Summary of Answers
(1) The graph of f(x) = –6/(x + 2) – 3 is that of a transformed reciprocal function with:
• a vertical asymptote x = –2,
• a horizontal asymptote y = –3,
• the point (–4, 0) (x-intercept) and (0, –6) (y-intercept),
and two branches which are reflections of each other about the point (–2, –3).
(2) Vertical asymptote: x = –2.
(3) Horizontal asymptote: y = –3.
(4) Domain: {x ∈ ℝ : x ≠ –2}.
(5) Range: {y ∈ ℝ : y ≠ –3}.
(6) Axes of symmetry: There are no lines of symmetry; however, the graph is symmetric with respect to the point (–2, –3).
Any sketch showing these features is correct.
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