The curve has equation , where and are constants. It is given that has a minimum point at .
- (i)
Show that and .
- (ii)
Sketch the graph of , indicating clearly the equations of any asymptotes and the coordinates of any turning points and axial intercepts.
- (iii)
By adding an appropriate graph to your sketch in (ii), deduce the number of real roots of the equation
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Worked solution
5(i)
1. Form an equation in $a$ and $b$ using the given point on the curve.
Since the curve passes through the minimum point , we substitute and into :
2. Find $\frac{dy}{dx}$ using the quotient rule.
3. Use the condition for a stationary point to form a second equation.
At the minimum point, the derivative is zero, so . Substituting :
4. Solve the simultaneous equations to find $a$ and $b$.
Adding equation (1) and equation (2):
Substitute into equation (1):
Thus, and (shown).
(shown)
5(ii)
1. Substitute the given values of $a$ and $b$ to find the equation of $C$ and its axial intercepts.
Substitute and from part (i):
For the -intercept, let :
For the -intercept, let :
The axial intercepts are and .
2. Find the equations of the vertical and oblique asymptotes.
By algebraic long division (or algebraic manipulation):
The vertical asymptote occurs where the denominator is zero:
As , . Therefore, the oblique asymptote is:
3. Determine the coordinates of the turning points analytically or via a graphing calculator.
Differentiating with respect to :
At the turning points, :
When , (this confirms the given minimum point).
When , .
The turning points are the minimum at and the maximum at .
4. Sketch the graph of $C$ incorporating all identified features.
Draw the axes and the asymptotes and .
Sketch the two branches of the rational curve:
- The upper right branch approaches as and as , passing through and turning at the local minimum .
- The lower left branch approaches as and as , turning at the local maximum .
Graph Sketch Requirements:
- Curve: Two distinct continuous branches appropriate for a rational function.
- Asymptotes: Vertical line and oblique line (drawn as dotted lines and labelled).
- Turning Points: Maximum point clearly indicated at and minimum point at .
- Axial Intercepts: Curve cuts the -axis at and touches the -axis at .
5(iii)
1. Rearrange the given cubic equation to express it in terms of the function for curve $C$.
Substitute and into the given equation:
Rearranging to isolate terms that match the numerator of :
Dividing both sides by (verifying that and are not roots):
The appropriate graph to add is .
2. Deduce the number of real roots by analyzing the intersections of the two graphs.
Imagine or sketch on the same axes as :
- For , the graph of is strictly non-negative () while , meaning no intersection.
- For , the graph of is entirely below the -axis (its local maximum is at ) while , meaning no intersection.
- For , as , and . As , and .
Since is continuous and monotonic decreasing on this interval while is continuous and monotonic increasing, the two curves cross exactly once in .
Therefore, there is point of intersection, meaning the original equation has real root.
1
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