The diagram shows the graph of y=f′(x)y = \text{f}'(x), which passes through the origin and has turning points at (2,5)(2, 5) and (6,0)(6, 0). The graph of y=f′(x)y = \text{f}'(x) has a vertical asymptote x=−1x = -1 and a horizontal asymptote y=5y = 5.

  1. (i)

    Sketch the graph of y=f′′(x)y = \text{f}''(x), stating the equation of asymptote(s) and the coordinates of any intersection(s) with the axes where possible.

  2. (ii)

    Write down the xx-coordinate(s) of the stationary point(s) of y=f(x)y = \text{f}(x) and determine their nature.

  3. (iii)

    If the tangent to the graph of y=f(x)y = \text{f}(x) at x=2x = 2 passes through the origin, find the equation of the normal to the graph of y=f(x)y = \text{f}(x) at x=2x = 2.

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Worked solution

4(i)

1. Identify the $x$-intercepts of the graph of $y = \text{f}''(x)$.

y=f′′(x)y = \text{f}''(x) represents the gradient function of y=f′(x)y = \text{f}'(x).
At the turning points of y=f′(x)y = \text{f}'(x), the gradient is zero.
Given the turning points are at (2,5)(2, 5) and (6,0)(6, 0), we have f′′(2)=0\text{f}''(2) = 0 and f′′(6)=0\text{f}''(6) = 0.
Thus, the xx-intercepts are (2,0)(2, 0) and (6,0)(6, 0).

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

As x→±∞x \to \pm\infty, y=f′(x)y = \text{f}'(x) approaches the horizontal asymptote y=5y = 5. This means the curve flattens out, so its gradient approaches 00. Thus, y=0y = 0 is a horizontal asymptote for y=f′′(x)y = \text{f}''(x).
The gradient of y=f′(x)y = \text{f}'(x) approaches +∞+\infty near the vertical asymptote x=−1x = -1, making x=−1x = -1 a vertical asymptote for y=f′′(x)y = \text{f}''(x) as well.
Equations of asymptotes: x=−1x = -1 and y=0y = 0.

3. Determine the sign and shape of $y = \text{f}''(x)$ in each interval.

  • For x<−1x < -1: f′(x)\text{f}'(x) is increasing towards +∞+\infty, so f′′(x)>0\text{f}''(x) > 0. It approaches y=0y=0 as x→−∞x \to -\infty and goes to +∞+\infty as x→−1−x \to -1^-.
  • For −1<x<2-1 < x < 2: f′(x)\text{f}'(x) is increasing, so f′′(x)>0\text{f}''(x) > 0. It comes from +∞+\infty, crosses the positive yy-axis, and meets the xx-axis at (2,0)(2, 0).
  • For 2<x<62 < x < 6: f′(x)\text{f}'(x) is decreasing, so f′′(x)<0\text{f}''(x) < 0. It reaches a local minimum and meets the xx-axis at (6,0)(6, 0).
  • For x>6x > 6: f′(x)\text{f}'(x) is increasing, so f′′(x)>0\text{f}''(x) > 0. It reaches a local maximum and approaches y=0y=0 as x→∞x \to \infty.
Answer

A sketch of y=f′′(x)y = \text{f}''(x) should include:

  • Asymptotes: x=−1x = -1 and y=0y = 0
  • Coordinates of intersections with axes: (2,0)(2, 0) and (6,0)(6, 0) (the yy-intercept is positive but its exact value cannot be determined).
  • Shape of the curve:
    • x<−1x < -1: Above the xx-axis, strictly increasing from the asymptote y=0y=0 to the asymptote x=−1x=-1.
    • −1<x<2-1 < x < 2: Decreasing from the asymptote x=−1x=-1, cutting the positive yy-axis, down to the xx-intercept (2,0)(2,0).
    • 2<x<62 < x < 6: Below the xx-axis, reaching a local minimum before crossing the xx-axis at (6,0)(6,0).
    • x>6x > 6: Above the xx-axis, reaching a local maximum before decreasing asymptotically towards y=0y=0.

4(ii)

1. Identify the $x$-coordinates of the stationary points of $\text{f}(x)$

Stationary points occur where f′(x)=0\text{f}'(x) = 0.
From the given graph of y=f′(x)y = \text{f}'(x), the graph intersects the xx-axis at the origin and at the turning point (6,0)(6, 0).
Therefore, f′(x)=0\text{f}'(x) = 0 at x=0x = 0 and x=6x = 6.

2. Determine the nature of the stationary point at $x = 0$

Just before x=0x = 0, f′(x)<0\text{f}'(x) < 0.
Just after x=0x = 0, f′(x)>0\text{f}'(x) > 0.
Since f′(x)\text{f}'(x) changes from negative to positive, there is a local minimum point at x=0x = 0.

3. Determine the nature of the stationary point at $x = 6$

Just before x=6x = 6, f′(x)>0\text{f}'(x) > 0.
Just after x=6x = 6, f′(x)>0\text{f}'(x) > 0.
Since f′(x)\text{f}'(x) does not change sign (remains positive), there is a point of inflexion at x=6x = 6.

Answer

x=0x = 0 (local minimum point) and x=6x = 6 (point of inflexion)

4(iii)

1. Determine the gradient of the tangent at $x = 2$ from the given graph of $\mathrm{f}'(x)$.

From the given graph of y=f′(x)y = \mathrm{f}'(x), there is a turning point at (2,5)(2, 5). This means that at x=2x = 2, the value of the derivative is f′(2)=5\mathrm{f}'(2) = 5.
Therefore, the gradient of the tangent at x=2x = 2 is 55.

2. Find the coordinates of the point of tangency on the graph of $y = \mathrm{f}(x)$.

The tangent passes through the origin (0,0)(0, 0) and has a gradient of m=5m = 5. Thus, the equation of the tangent line is y=5xy = 5x.
Since the tangent touches the graph of y=f(x)y = \mathrm{f}(x) at x=2x = 2, we can substitute x=2x = 2 into the tangent equation to find the yy-coordinate of the point of tangency:
y=5(2)=10y = 5(2) = 10
The point of contact on the curve is (2,10)(2, 10).

3. Determine the gradient of the normal and find its equation.

The gradient of the normal is the negative reciprocal of the gradient of the tangent:
mnormal=−1f′(2)=−15m_{\text{normal}} = -\frac{1}{\mathrm{f}'(2)} = -\frac{1}{5}
Using the point (2,10)(2, 10), the equation of the normal is:
y−10=−15(x−2)y - 10 = -\frac{1}{5}(x - 2)
y=−15x+25+10y = -\frac{1}{5}x + \frac{2}{5} + 10
y=−15x+525y = -\frac{1}{5}x + \frac{52}{5}

Answer

y=−15x+525y = -\frac{1}{5}x + \frac{52}{5}

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