The diagram below shows the graph of y=f(x)y = \text{f}(x). The curve passes through the xx-axis at (−2,0)(-2, 0) and (1,0)(1, 0), and has a maximum point with coordinates (2,2)(2, 2). The lines x=−1x = -1 and y=1y = 1 are asymptotes to the graph. Stating the equations of any asymptotes and the coordinates of any points of intersection with the axes and stationary points, where possible, sketch the graphs of

  1. (a)

    y=3f(x+a)y = 3\text{f}(x + a), where aa is a positive constant such that 1<a<21 < a < 2,

  2. (b)

    y=f′(x)y = \text{f}'(x).

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Worked solution

2(a)

1. Identify the geometrical transformations required

The equation y=3f(x+a)y = 3\text{f}(x + a) indicates two transformations on the graph of y=f(x)y = \text{f}(x):

  1. A translation by aa units in the negative xx-direction (shift left by aa).
  2. A vertical stretch by a scale factor of 33 parallel to the yy-axis.

2. Find the equations of the new asymptotes

Original vertical asymptote: x=−1x = -1
Translated left by aa:
x=−1−ax = -1 - a

Original horizontal asymptote: y=1y = 1
Stretched vertically by 3:
y=1×3  ⟹  y=3y = 1 \times 3 \implies y = 3

3. Find the coordinates of the new stationary point

Original maximum point: (2,2)(2, 2)
Translated left by aa and stretched vertically by 3:
(x,y)→(2−a,2×3)  ⟹  (2−a,6)(x, y) \to (2 - a, 2 \times 3) \implies (2 - a, 6)
Since 1<a<21 < a < 2, we have 0<2−a<10 < 2 - a < 1, so the new maximum point lies in the first quadrant.

4. Determine the new intersections with the axes

Original xx-intercepts: (−2,0)(-2, 0) and (1,0)(1, 0)
Translated left by aa (vertical stretch does not affect xx-intercepts):
(−2−a,0)and(1−a,0)(-2 - a, 0) \quad \text{and} \quad (1 - a, 0)
Since 1<a<21 < a < 2, we have −1<1−a<0-1 < 1 - a < 0, meaning both xx-intercepts lie on the negative xx-axis.

The new yy-intercept occurs at x=0x = 0, giving y=3f(a)y = 3\text{f}(a). From the original graph, for 1<x<21 < x < 2, f(x)>0\text{f}(x) > 0. Hence f(a)>0\text{f}(a) > 0, and the curve intersects the positive yy-axis (the exact coordinate cannot be determined).

5. Sketch the final transformed graph

Sketch the two branches based on the transformed features:

  • Left branch: Bounded by x=−1−ax = -1 - a and y=3y = 3, passing through (−2−a,0)(-2-a, 0).
  • Right branch: Starts near the bottom of x=−1−ax = -1 - a, crosses the xx-axis at (1−a,0)(1-a, 0), crosses the positive yy-axis, peaks at the maximum (2−a,6)(2-a, 6), and approaches y=3y = 3 from above as x→∞x \to \infty.
Answer

Sketch Features:

  • Shape: Two branches similar to y=f(x)y = \text{f}(x).
  • Asymptotes: Vertical line x=−1−ax = -1 - a and horizontal line y=3y = 3.
  • Axes Intersections: xx-intercepts at (−2−a,0)(-2-a, 0) and (1−a,0)(1-a, 0). The graph intersects the positive yy-axis.
  • Stationary Point: Maximum point at (2−a,6)(2-a, 6).

2(b)

1. Determine the equations of the asymptotes for $y = \text{f}'(x)$.

Since the graph of y=f(x)y = \text{f}(x) has a vertical asymptote at x=−1x = -1, the gradient is undefined and its magnitude approaches infinity. Thus, x=−1x = -1 is a vertical asymptote for y=f′(x)y = \text{f}'(x).
As x→±∞x \to \pm \infty, the curve approaches the horizontal asymptote y=1y = 1, meaning the curve flattens out and the gradient approaches 00. Thus, y=0y = 0 (the xx-axis) is a horizontal asymptote for y=f′(x)y = \text{f}'(x).

2. Determine the coordinates of intersection with the axes.

At the maximum point (2,2)(2, 2) on y=f(x)y = \text{f}(x), the gradient is 00. Therefore, f′(2)=0\text{f}'(2) = 0, giving an xx-intercept at (2,0)(2, 0).
The exact yy-intercept cannot be calculated from the given information, but it must be positive since f(x)\text{f}(x) is strictly increasing at x=0x = 0.

3. Analyse the sign of the gradient in each interval to deduce the shape of the derivative graph.

For x<−1x < -1, f(x)\text{f}(x) is strictly decreasing, so f′(x)<0\text{f}'(x) < 0. The curve comes from y=0y = 0 as x→−∞x \to -\infty and plunges to −∞-\infty as x→−1−x \to -1^-.
For −1<x<2-1 < x < 2, f(x)\text{f}(x) is strictly increasing, so f′(x)>0\text{f}'(x) > 0. The curve falls from ∞\infty as x→−1+x \to -1^+, and decreases to cross the xx-axis at (2,0)(2, 0).
For x>2x > 2, f(x)\text{f}(x) is strictly decreasing, so f′(x)<0\text{f}'(x) < 0. The curve dips below the xx-axis. Since it must approach the horizontal asymptote y=0y = 0 as x→∞x \to \infty, it must reach a local minimum point before turning upwards. (The exact coordinates of this minimum cannot be determined).

Answer

Asymptotes: x=−1x = -1, y=0y = 0
Points of intersection: (2,0)(2, 0) on the xx-axis. (The exact yy-intercept cannot be determined, but must be plotted as positive).
Stationary points: A local minimum exists for some x>2x > 2 (exact coordinates cannot be determined).

Sketch instructions:

  • Left branch (x<−1x < -1): Drawn entirely below the xx-axis. Approaches y=0y = 0 from below as x→−∞x \to -\infty, and approaches x=−1x = -1 going towards −∞-\infty.
  • Right branch (x>−1x > -1): Comes down from ∞\infty near x=−1+x = -1^+, crosses the yy-axis at a positive value, crosses the xx-axis at (2,0)(2, 0), reaches a local minimum (with a negative yy-value), and curves back up to approach y=0y = 0 from below as x→∞x \to \infty.

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