The curve has equation , where .
- (i)
Find the equation of the tangent at the point where cuts the -axis.
- (ii)
Sketch , giving the equations of asymptotes, the coordinates of turning points and axial intercepts in terms of , if any.
- (iii)
Find the equation of the normal at the point where cuts the -axis. Leave your answer in terms of .
- (iv)
Hence show that the value of the area bounded by the tangent at , the normal at and both the - and -axes is more than square units.
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Worked solution
7(i)
1. Find the coordinates of point $A$ where the curve cuts the $y$-axis.
Substitute into the equation of :
Point is .
2. Find the gradient of the tangent at $A$ by finding $\frac{\mathrm{d}y}{\mathrm{d}x}$ and evaluating it at $x = 0$.
Using the quotient rule:
Substitute :
The gradient of the tangent at is .
3. State the equation of the tangent at point $A$.
Since the gradient is and the line passes through , the tangent is a horizontal line.
7(ii)
1. Determine the axial intercepts of the curve.
To find the -intercept, set :
Thus, the -intercept is .
To find the -intercept, set :
Thus, the -intercept is .
2. Find the equations of the horizontal and vertical asymptotes.
For the horizontal asymptote, we consider the behavior as :
So, the horizontal asymptote is .
For vertical asymptotes, the denominator approaches zero:
Using the quadratic formula to solve for :
These are the equations of the vertical asymptotes.
3. Differentiate the equation to find the $x$-coordinates of the turning points.
Using the quotient rule :
For turning points, set :
Since (meaning ), the possible solutions are:
4. Calculate the corresponding $y$-coordinates to finalize the turning points.
When :
When :
So, the coordinates of the turning points are and .
(The student would then synthesize these features to sketch the graph, with as a local minimum and as a local maximum, properly bound by the respective asymptotes).
Axial Intercepts:
-intercept:
-intercept:
Equations of Asymptotes:
Horizontal asymptote:
Vertical asymptotes:
Turning Points:
and
(The required sketch of should show the curve passing through these intercepts and turning points, guided by the stated asymptotes.)
7(iii)
1. Find the coordinates of point $B$ where the curve intersects the $x$-axis.
At the -axis, .
The coordinates of are .
2. Differentiate $y$ with respect to $x$ to find the gradient function.
Using the quotient rule:
3. Evaluate the gradient of the normal at $B$ and find its equation.
Substitute into the derivative to find the gradient of the tangent, :
The gradient of the normal, , is :
Using the point-gradient form for :
7(iv)
1. State or determine the equations of the tangent at $A$ and the normal at $B$ from earlier parts.
From earlier parts, cuts the -axis at and the -axis at .
At , , so the equation of the tangent is .
At , . The gradient of the normal is .
Equation of the normal at : .
2. Find the point of intersection between the tangent at $A$ and the normal at $B$.
Substitute into the normal equation:
The intersection point is .
3. Calculate the area of the region bounded by the tangent, the normal, and both axes.
The region is bounded by , , (-axis), and (-axis).
This forms a right-angled trapezium with vertices , , , and .
The parallel horizontal sides lie on and with lengths and respectively. The height along the -axis is .
4. Use the given condition $k > 1$ to show that the area is strictly greater than $\frac{15}{8}$.
We are given that .
Multiplying by reverses the inequality:
Therefore, adding to both sides:
Thus, the area is more than square units.
Area . Since , Area square units. (Shown)
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