The curve has the equation where , , and are real constants. It is given that the equations of asymptotes of are and . Also, passes through the point .
- (i)
Explain why the value of is 1. State the value of and show that .
- (ii)
Sketch the curve , indicating clearly the equations of asymptotes, axial intercepts and stationary points.
- (iii)
Find the exact equation of the normal to at the point .
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Worked solution
4(i)
1. Explain why $a = 1$ using the oblique asymptote.
The rational function is .
Performing polynomial long division, the quotient begins with the term .
Since the oblique asymptote is given as , the linear quotient must be .
Comparing the coefficient of , we deduce that .
2. State the value of $c$ by identifying the vertical asymptote.
The vertical asymptote occurs where the denominator is zero, so .
Given that the vertical asymptote is , we have:
3. Show that $b = 3$ by substituting the given point $P$.
The curve passes through the point .
Substituting , , and into the curve equation :
and (shown)
4(ii)
1. Determine the axial intercepts of the curve.
From part (i), , , and , so the equation of the curve is:
To find the -intercept, let :
The point is the -intercept (also given as point ).
To find any -intercepts, let :
Checking the discriminant, .
Since , there are no real roots, meaning the curve has no -intercepts.
2. Find the coordinates of the stationary points.
Using the asymptotes and , or by algebraic division, can be rewritten as:
Differentiating with respect to :
At stationary points, :
Substitute the -values back into the equation of the curve to find the corresponding -coordinates:
When , .
When , .
Therefore, the stationary points are and .
3. Sketch the curve with all indicated key features.
Draw the coordinate axes.
- Draw and label the vertical asymptote and oblique asymptote with dashed lines.
- Mark the -intercept at .
- Plot and label the local maximum turning point at and the local minimum turning point at .
- Sketch the two branches of the curve such that they pass through the respective points and asymptotically approach the dashed lines.
A valid sketch must clearly show:
- Two branches of the hyperbola.
- Asymptotes drawn as dashed lines and labelled as and .
- The -intercept plotted and labelled at .
- Two stationary points plotted and labelled at and .
4(iii)
1. Find the derivative $\frac{dy}{dx}$ of the curve.
From part (i), , and , so the equation of the curve is:
Using the quotient rule:
2. Calculate the gradient of the tangent at $P\left(0, -\frac{3}{2}\right)$.
Substitute into :
3. Find the gradient of the normal and the exact equation of the normal.
The gradient of the normal is the negative reciprocal of the tangent gradient:
Using the point-gradient form for :
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