[In this question, sketches of the given graphs are not drawn to scale] The graphs of y=∣f(x)∣y = |\mathrm{f}(x)| and y2=f(x)y^2 = \mathrm{f}(x) are given below. On separate diagrams, draw sketches of the graphs of stating the equations of any asymptotes and the coordinates of any points of intersection with the axes.

  1. (a)

    y2=f(−2x)y^2 = \mathrm{f}(-2x),

  2. (b)

    y=f(x)y = \mathrm{f}(x),

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Worked solution

1(a)

1. Identify the transformation from $y^2 = \mathrm{f}(x)$ to $y^2 = \mathrm{f}(-2x)$.

The equation y2=f(−2x)y^2 = \mathrm{f}(-2x) replaces xx with −2x-2x. This corresponds to a horizontal scaling by a factor of 12\frac{1}{2} and a reflection in the yy-axis.

2. Determine the new asymptote and $x$-intercept.

Applying the transformation to the key features of y2=f(x)y^2 = \mathrm{f}(x):

  • Vertical asymptote: x=3  ⟹  −2x=3  ⟹  x=−32x = 3 \implies -2x = 3 \implies x = -\frac{3}{2}
  • xx-intercept (vertex): x=5  ⟹  −2x=5  ⟹  x=−52x = 5 \implies -2x = 5 \implies x = -\frac{5}{2}, giving the coordinate (−52,0)(-\frac{5}{2}, 0)

3. Determine the shape and domain of the new graph.

The original graph exists for 3<x≤53 < x \le 5 and opens to the left from (5,0)(5, 0).
For the transformed graph, the domain is 3<−2x≤5  ⟹  −52≤x<−323 < -2x \le 5 \implies -\frac{5}{2} \le x < -\frac{3}{2}.
The new graph consists of two branches starting at (−52,0)(-\frac{5}{2}, 0) and opening to the right towards the vertical asymptote x=−32x = -\frac{3}{2}, maintaining symmetry about the xx-axis.

Answer

Sketch of y2=f(−2x)y^2 = \mathrm{f}(-2x):

  • A curve symmetric about the xx-axis consisting of two branches.
  • The vertex (xx-intercept) is at (−52,0)(-\frac{5}{2}, 0).
  • The curve opens to the right from the vertex.
  • There is a vertical asymptote at x=−32x = -\frac{3}{2}, which the branches approach as y→±∞y \to \pm\infty.
  • No yy-intercepts.

1(b)

1. Determine the intervals where $\mathrm{f}(x)$ is positive or negative from the $y^2 = \mathrm{f}(x)$ graph.

The relation y2=f(x)y^2 = \mathrm{f}(x) only yields real values of yy when f(x)≥0\mathrm{f}(x) \ge 0.
Given that the graph of y2=f(x)y^2 = \mathrm{f}(x) consists of branches starting at (5,0)(5,0) and opening to the left towards the asymptote x=3x=3, this curve only exists for 3<x≤53 < x \le 5.
Furthermore, it is explicitly defined "only for x>3x > 3", ruling out any regions x<3x < 3.
Therefore, we deduce:

  • f(x)≥0\mathrm{f}(x) \ge 0 for 3<x≤53 < x \le 5
  • f(x)≤0\mathrm{f}(x) \le 0 for x<3x < 3 and x>5x > 5

2. Relate $y = \mathrm{f}(x)$ to the provided graph of $y = |\mathrm{f}(x)|$.

Using the signs of f(x)\mathrm{f}(x), we construct y=f(x)y = \mathrm{f}(x) from y=∣f(x)∣y = |\mathrm{f}(x)|:

  • For 3≤x≤53 \le x \le 5: Since f(x)≥0\mathrm{f}(x) \ge 0, f(x)=∣f(x)∣\mathrm{f}(x) = |\mathrm{f}(x)|. The sketch of y=f(x)y = \mathrm{f}(x) in this region is identical to the graph of y=∣f(x)∣y = |\mathrm{f}(x)|.
  • For x<3x < 3 and x>5x > 5: Since f(x)≤0\mathrm{f}(x) \le 0, f(x)=−∣f(x)∣\mathrm{f}(x) = -|\mathrm{f}(x)|. The sketch is obtained by reflecting the corresponding parts of the graph of y=∣f(x)∣y = |\mathrm{f}(x)| in the xx-axis.

Note that the sharp cusp on y=∣f(x)∣y = |\mathrm{f}(x)| at x=5x = 5 unfolds into a smooth curve crossing the xx-axis in y=f(x)y = \mathrm{f}(x), which matches f(x)\mathrm{f}(x) changing sign from positive to negative.

3. Determine the exact asymptotes and intercepts.

Applying the reflections in the xx-axis for x<3x < 3 and x>5x > 5 to the key features of the ∣f(x)∣|\mathrm{f}(x)| graph:

  • The horizontal asymptote y=1y = 1 reflects to become y=−1y = -1.
  • The yy-intercept (0,2)(0, 2) reflects to become (0,−2)(0, -2).
  • The known xx-intercept (5,0)(5, 0) remains (5,0)(5, 0).
  • The smooth local minimum touching the xx-axis at the negative root remains an xx-intercept, becoming a local maximum.
  • The vertical asymptotes x=1x = 1 and x=3x = 3 are unchanged.
Answer

Sketch of y=f(x)y = \mathrm{f}(x):
[A valid sketch should show the following features on a coordinate plane]

  • x<1x < 1: Curve approaching y=−1y = -1 from above as x→−∞x \to -\infty, rising to touch the xx-axis at a negative xx-value (smooth local maximum), falling to cross the yy-axis at (0,−2)(0, -2), and tending to −∞-\infty as x→1−x \to 1^-.
  • 1<x<31 < x < 3: An inverted U-shaped curve entirely below the xx-axis, tending to −∞-\infty as x→1+x \to 1^+ and as x→3−x \to 3^-.
  • x>3x > 3: Curve starting from +∞+\infty near x=3+x = 3^+, falling to cross the xx-axis smoothly at (5,0)(5, 0), and continuing to decrease while approaching y=−1y = -1 from above as x→∞x \to \infty.

Equations of Asymptotes:
x=1x = 1, x=3x = 3, y=−1y = -1

Coordinates of Points of Intersection:
yy-axis: (0,−2)(0, -2)
xx-axis: (5,0)(5, 0) and an unknown negative coordinate (x0,0)(x_0, 0).

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