The diagram shows the graph of , which passes through the origin and has turning points at and . The graph of has a vertical asymptote and a horizontal asymptote .
- (i)
Sketch the graph of , stating the equation of asymptote(s) and the coordinates of any intersection(s) with the axes where possible.
- (ii)
Write down the -coordinate(s) of the stationary point(s) of and determine their nature.
- (iii)
If the tangent to the graph of at passes through the origin, find the equation of the normal to the graph of at .
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Worked solution
4(i)
1. Identify the $x$-intercepts of the graph of $y = \text{f}''(x)$.
represents the gradient function of .
At the turning points of , the gradient is zero.
Given the turning points are at and , we have and .
Thus, the -intercepts are and .
2. Determine the equations of the asymptotes for $y = \text{f}''(x)$.
As , approaches the horizontal asymptote . This means the curve flattens out, so its gradient approaches . Thus, is a horizontal asymptote for .
The gradient of approaches near the vertical asymptote , making a vertical asymptote for as well.
Equations of asymptotes: and .
3. Determine the sign and shape of $y = \text{f}''(x)$ in each interval.
- For : is increasing towards , so . It approaches as and goes to as .
- For : is increasing, so . It comes from , crosses the positive -axis, and meets the -axis at .
- For : is decreasing, so . It reaches a local minimum and meets the -axis at .
- For : is increasing, so . It reaches a local maximum and approaches as .
A sketch of should include:
- Asymptotes: and
- Coordinates of intersections with axes: and (the -intercept is positive but its exact value cannot be determined).
- Shape of the curve:
- : Above the -axis, strictly increasing from the asymptote to the asymptote .
- : Decreasing from the asymptote , cutting the positive -axis, down to the -intercept .
- : Below the -axis, reaching a local minimum before crossing the -axis at .
- : Above the -axis, reaching a local maximum before decreasing asymptotically towards .
4(ii)
1. Identify the $x$-coordinates of the stationary points of $\text{f}(x)$
Stationary points occur where .
From the given graph of , the graph intersects the -axis at the origin and at the turning point .
Therefore, at and .
2. Determine the nature of the stationary point at $x = 0$
Just before , .
Just after , .
Since changes from negative to positive, there is a local minimum point at .
3. Determine the nature of the stationary point at $x = 6$
Just before , .
Just after , .
Since does not change sign (remains positive), there is a point of inflexion at .
(local minimum point) and (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 , there is a turning point at . This means that at , the value of the derivative is .
Therefore, the gradient of the tangent at is .
2. Find the coordinates of the point of tangency on the graph of $y = \mathrm{f}(x)$.
The tangent passes through the origin and has a gradient of . Thus, the equation of the tangent line is .
Since the tangent touches the graph of at , we can substitute into the tangent equation to find the -coordinate of the point of tangency:
The point of contact on the curve is .
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:
Using the point , the equation of the normal is:
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