Asymptotes as the lines y=x an y=-x

  • Thread starter Dogtanian
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Hi there.

I'm a maths teacher and today was having a discussion with my Head of Department about asymptotes (as you do!)

She was me if I could think on an equation of a graph(s) which has an asymptotes at the line y=x and another at y=-x.

Thinking about it, neither of us could come up with anything. It's really bugging me now. I ain't too sure if there is such a function...though it seems silly there not being.

The only way I managed to get a graph like what I wanted was to rotate the graph of y=1/x by various angles.

So can such a graph exist? Is it possible to transform equations of graphs via rotations (something I can't ever remember doing)?

Hope I can come up with an answer by tomorrow :)
 

Answers and Replies

How about

[tex]y = x \tanh{x}[/tex] ?

as

[tex] x \tanh{x} = x \frac{e^x - e^{-x}}{e^x + e^{-x}}[/tex]

therefore, for large positive x,

[tex]x \tanh{x} \approx x \frac{e^x}{e^x} = x[/tex]

and for large negative x

[tex] x \tanh{x} \approx x (\frac{-e^{-x}}{e^{-x}}) = -x[/tex]
 
arildno
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Dogtanian said:
Hi there.

I'm a maths teacher and today was having a discussion with my Head of Department about asymptotes (as you do!)

She was me if I could think on an equation of a graph(s) which has an asymptotes at the line y=x and another at y=-x.

Thinking about it, neither of us could come up with anything. It's really bugging me now. I ain't too sure if there is such a function...though it seems silly there not being.

The only way I managed to get a graph like what I wanted was to rotate the graph of y=1/x by various angles.

So can such a graph exist? Is it possible to transform equations of graphs via rotations (something I can't ever remember doing)?

Hope I can come up with an answer by tomorrow :)
You certainly can rotate curves!

For example, the curve described by the equation:
[tex]x^{2}-y^{2}=1[/tex]
has asymptotes y=x and y=-x.

Note, however, that in this case, there exists no function of x, by which the y-coordinates of the curve could be computed, and the curve in question cannot be regarded as the graph of some function, i.e, the set of points describable as (x,f(x)), where f is some function.
 
robphy
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Doesn't one branch of that hyperbola still have those lines as asymptotes?
In a physics context, the worldline of a uniformly accelerated observer [which can be regarded as function (t,x(t)), where x(t)=sqrt(1+t^2)] is asymptotic to a light cone.
 
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Thanks for the help guys :D

My head of department was so sure you could each of the 4 sections between the said asymptotes filled with a curve.

Using what the first two posts said, I drew the graphs of

y = sqrt(1+x^2)
y = -sqrt(1+x^2)
x = sqrt(1+y^2)
x = -sqrt(1+y^2)

to get what I needed (at least I think that is what I i if I remember correctly back to last night....)

But I only went and forgot all about this today until just now...so I never i speak to my HofD about it...never mind :D
 
Hurkyl
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You can combine the four curves to get one that fills the whole thing. :smile:

We already know that

x² - y² = 1

fills in two sections, and

y² - x² = 1

fills in the other two.

We can combine them into one equation:

(x² - y² - 1) (y² - x² - 1) = 0

which will fill in all four quadrants.
 
robphy
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...which can be simplified to read (x² - y²)² = 1.
Of course, the easy way to see this is to note that
y² - x² = 1 is the same as x² - y² = -1.
 

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