A curve has parametric equations , .
- (a)
Sketch the curve, stating the exact coordinates of any points of intersection with the axes and the equations of any asymptotes.
- (b)
Find the equation of the normal to the curve at the point .
- (c)
The normal at the point meets the x-axis at and the y-axis at . Find the value of the ratio , where is the origin, giving your answer in terms of .
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Worked solution
7(a)
1. Find the coordinates of the intersection with the axes.
For the y-intercept, set :
Substitute to find :
The point of intersection with the y-axis is .
For the x-intercept, set :
Since for all real , there are no real solutions. Thus, the curve does not intersect the x-axis.
2. Determine the equations of any asymptotes.
Consider the behavior of the curve as becomes very large in magnitude ():
As , the x-coordinate .
At the same time, the y-coordinate .
Therefore, the line is a horizontal asymptote.
3. Sketch the curve based on its properties (or via its Cartesian equation).
To confirm the shape, we can find the Cartesian equation by substituting :
The graph is an even function (symmetric about the y-axis), passing through the maximum point and forming a bell shape that approaches as .
(The student draws this bell-shaped curve on the axes, labelling the intercept and the asymptote .)
Intersection with axes:
Asymptote:
(The sketch is a bell-shaped curve with a maximum point at on the y-axis, symmetric about the y-axis, and asymptotic to the x-axis in both the positive and negative x directions.)
7(b)
1. Differentiate parametrically to find the gradient of the tangent at $t = p$.
At the point , the parameter is .
So the gradient of the tangent is .
2. Find the gradient of the normal and formulate the equation of the normal.
The gradient of the normal is .
Using the point-slope form, the equation of the normal at is:
7(c)
1. Find the parameter $t$ and the equation of the normal at point $C$.
At point , we use the x-coordinate to find :
Using the derivatives from the parametric equations:
At , the gradient of the tangent is:
The gradient of the normal is the negative reciprocal of the tangent gradient:
Using the point-slope form, the equation of the normal at is:
2. Find the coordinates of intersection points $A$ and $B$.
The normal meets the x-axis at . Set :
Thus, is .
The normal meets the y-axis at . Set :
Thus, is .
3. Calculate the distances $|OA|$ and $|OB|$ to deduce the ratio.
The distance is the magnitude of the x-coordinate of :
The distance is the magnitude of the y-coordinate of :
We can factor out from to compare with :
The required ratio is:
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