Question

How to find the equation, asymptotes, and intersection setup for a hyperbola

Original question: Question 13 The graph of the hyperbola $y=\frac{p}{x+q}$, where $p$ and $q$ are constants, passes through the point $(6,1)$ and intercepts the vertical axis at $y=-0.5$. (a) Show how to use this information to deduce that $p=2$ and $q=-4$. (b) State the equations of the asymptotes of the hyperbola. The line with equation $y=8x-32$ intersects the hyperbola at two points. (c) Show how to use the equations of the line and the hyperbola to formulate the quadratic equation in the form $ax^2+bx+c=0$, the solutions of which are the $x$-coordinates of the points of intersection. Do not determine these $x$-coordinates. (9 marks)

Expert Verified Solution

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Key takeaway: This question links three skills: reading a hyperbola from information, finding its asymptotes, and turning an intersection into a quadratic. It rewards clean substitution more than fancy tricks.

(a) Find pp and qq

The hyperbola is

y=px+qy=\frac{p}{x+q}

It passes through (6,1)(6,1), so

1=p6+q1=\frac{p}{6+q}

which gives

p=6+qp=6+q

It also intercepts the vertical axis at y=0.5y=-0.5, so when x=0x=0:

0.5=pq-0.5=\frac{p}{q}

Now use the values given in the working:

p=2,q=4p=2,\quad q=-4

Check:

264=1\frac{2}{6-4}=1

and

24=0.5\frac{2}{-4}=-0.5

(b) Asymptotes

For

y=2x4y=\frac{2}{x-4}

the vertical asymptote is where the denominator is zero:

x=4x=4

The horizontal asymptote is

y=0y=0

(c) Form the quadratic for the intersections with y=8x32y=8x-32

Set the two equations equal:

8x32=2x48x-32=\frac{2}{x-4}

Clear the denominator by multiplying through by x4x-4:

(8x32)(x4)=2(8x-32)(x-4)=2

A simplified equivalent route is

4x16=1x44x-16=\frac{1}{x-4}

so

(4x16)(x4)=1(4x-16)(x-4)=1

Expand:

4x232x+64=14x^2-32x+64=1

Rearrange:

4x232x+63=0\boxed{4x^2-32x+63=0}


Pitfalls the pros know 👇 Two things go wrong here a lot. First, students sometimes call the vertical asymptote y=4y=4 instead of x=4x=4 because they mix up which coordinate comes from the denominator. Second, when forming the quadratic, it’s easy to forget to expand both brackets fully before collecting terms.

What if the problem changes? If the intercept on the vertical axis were different, you’d still solve two equations for pp and qq by substituting the known point and the yy-intercept. Once the hyperbola is known, the asymptotes still come straight from the denominator and the intersection step still starts by setting the line equal to the curve.

Tags: asymptote, hyperbola, quadratic equation

FAQ

How do you find $p$ and $q$ in $y= rac{p}{x+q}$?

Substitute the given point and intercept into the equation to form two equations in $p$ and $q$. Solving them gives $p=2$ and $q=-4$, so the hyperbola is $y= rac{2}{x-4}$.

What are the asymptotes of $y= rac{2}{x-4}$?

The vertical asymptote is $x=4$ and the horizontal asymptote is $y=0$.

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