The curve has equation , where is a constant.
- (i)
Find the range of values of if has two distinct stationary points.
- (ii)
For the rest of this question, use .
Sketch the graph of , indicating clearly the equations of any asymptotes and the coordinates of any turning points and axial-intercepts.
- (iii)
Using your sketch in (ii), find the range of values of , where is a positive real number for which the equation
has no real roots.
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Worked solution
4(i)
1. Find the first derivative of $y$ with respect to $x$ using the quotient rule.
Let .
Using the quotient rule:
2. Equate the derivative to zero and establish the condition for two distinct stationary points.
For stationary points, we set :
For the curve to have two distinct stationary points, this quadratic equation must have two distinct real roots (and the roots must not coincide with the vertical asymptote ).
3. Apply the discriminant condition to find the corresponding range of $k$.
For two distinct real roots, the discriminant :
(Note: Checking in the numerator gives . Since our range restricts , will never be . Thus, the roots are valid stationary points.)
4(ii)
1. Substitute $k = 4$ and rewrite the expression to find the equations of the asymptotes.
Substitute into the equation of :
Perform algebraic division (or rewrite the numerator):
As , . Thus, the oblique asymptote is .
As , . Thus, the vertical asymptote is .
2. Find the coordinates of the axial-intercepts.
To find the -intercept, substitute :
The -intercept is .
To find the -intercept, substitute :
The -intercept is .
3. Find the coordinates of the turning points.
Using the rewritten form , differentiate with respect to :
Set to find stationary points:
Substitute back to find -coordinates:
When , .
When , .
The turning points are and .
4. Sketch the graph of $C$ incorporating all the calculated features.
Draw axes and label the origin.
Draw the dotted line and the dotted line , labelling them.
Plot the minimum turning point at on the upper-right branch.
Plot the maximum turning point at on the lower-left branch.
Draw the two distinct branches of the hyperbola approaching the respective asymptotes.
(A graph sketch is required. Key features to include):
- Vertical asymptote:
- Oblique asymptote:
- Minimum turning point / -intercept:
- Maximum turning point / -intercept:
4(iii)
1. Rearrange the given equation to relate it to the curve $C$ and a straight line.
Given the equation for :
Dividing by , we obtain:
This equation models the -coordinates of the intersections between the curve : and the straight line : .
2. Identify the geometric properties of the line $L$ relative to the sketch of $C$.
The line always passes through the point with gradient .
From the sketch in (ii), the oblique asymptote of is , which also passes through and has a gradient of .
3. Use the gradients from the sketch to deduce the range of $m$ for no intersections.
For the equation to have no real roots, the line must not intersect the curve .
Pivoting around , if the line is steeper than the asymptote (), it will cross into the region containing the upper right branch of and intersect it.
If the line is less steep than or parallel to the asymptote (), it remains in the empty region bounded between the two branches of and its asymptotes, thus avoiding intersection.
Since the question specifies is a positive real number (), the required range is:
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