The curve C is defined by the parametric equations for . Another curve D has the following parametric equations. for and .
- (a)
Find the coordinates of the point on the curve C corresponding to .
- (b)
Find the cartesian equation for curve D.
- (c)
Sketch the curves C and D on the same diagram.
- (d)
Hence determine the range of values of k such that the equation
has no real solutions.
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Worked solution
4(a)
1. Substitute $t = 0$ into the parametric equations of curve C to find the corresponding $x$ and $y$ values.
Substitute into the equation for :
Substitute into the equation for :
The coordinates are .
\left( \frac{3}{2}, 0 \right)
4(b)
1. Express $\cos \theta$ and $\sin \theta$ in terms of $x$ and $y$ respectively.
2. Use the Pythagorean identity to eliminate the parameter $\theta$ and obtain the Cartesian equation.
Using the identity :
4(c)
1. Sketch curve C by interpreting the given parametric equations using a graphing calculator.
Using a graphing calculator, plot the parametric equations for curve C: , for .
The calculator will display a self-intersecting (looped) curve strictly within the first quadrant. The starting point at is and the ending point at is . Mark these coordinates on the axes.
2. Sketch curve D using the standard properties of its cartesian equation.
From part (b), the cartesian equation for curve D is .
This represents an ellipse centered at .
The vertical semi-axis length is , giving the highest point and lowest point . Note that the highest point aligns horizontally with the endpoint of curve C.
The horizontal semi-axis length is , giving the -intercepts and .
Draw a representative full ellipse for an arbitrary , labelling these key points.
A sketch containing:
- Curve C: A looped curve in the first quadrant starting at and ending at .
- Curve D: A full ellipse centered at , passing through and , with horizontal extremities at and .
4(d)
1. Link the given equation to the intersection of Curve C and Curve D.
Substitute the parametric equations of Curve C, and , into the given equation:
This matches the cartesian equation of Curve D found in part (b). Thus, the given equation represents points where Curve C intersects Curve D. For the equation to have no real solutions, the two curves must not intersect.
2. Determine the maximum $x$-coordinate of Curve C.
From part (a), the starting point of Curve C (at ) is .
Since and , the maximum possible value for on Curve C is:
Thus, Curve C lies entirely in the region , connecting to .
3. Determine the minimum $x$-coordinate of Curve D.
Curve D is an ellipse centered at with a horizontal semi-axis of and a vertical semi-axis of .
The left-most point (minimum -coordinate) of the ellipse is .
4. Establish the geometric condition for no intersection and solve for $k$.
For the curves to not intersect, Curve D must lie strictly to the right of Curve C. We require the minimum of Curve D to be strictly greater than the maximum of Curve C:
(Note: If , the point of Curve C lies inside or on the ellipse. Since the highest point of Curve C, , is always outside the ellipse — as 's maximum is occurring exclusively at — Curve C will inevitably cross Curve D, guaranteeing real solutions.)
Given the condition , the range of values is:
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