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#1 2012-10-21 12:16:39

skylor blay
Guest

find domain and asymptotes and y intercept

I have to find the domain, vertical and horizontal asymptotes, and the y-intercept and graph it for.

f(x) 3x^2 + 4/x^2-x-6 and

g(x) = 2x-15/x^2-8x+15

I was marked 4/8 but wasn't told what I did wrong.

#2 2012-10-21 16:20:54

bobbym
Administrator
From: Bumpkinland
Registered: 2009-04-12
Posts: 90,606

Re: find domain and asymptotes and y intercept

Hi;

Says there you also have to graph it. Did you go here and do that?

http://www.mathsisfun.com/data/function … =-8&ymax=8


In mathematics, you don't understand things. You just get used to them.

I agree with you regarding the satisfaction and importance of actually computing some numbers. I can't tell you how often I see time and money wasted because someone didn't bother to run the numbers.

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#3 2012-10-21 16:34:44

skylor
Guest

Re: find domain and asymptotes and y intercept

I believe my graph is correct. I will try it out.

#4 2012-10-21 16:52:45

skylor
Guest

Re: find domain and asymptotes and y intercept

I'm also pretty sure both of the asymptotes are correct, not so sure about the domain and y intercept. The graph is also kind of sloppy.

#5 2012-10-21 20:15:43

bob bundy
Moderator
Registered: 2010-06-20
Posts: 6,531

Re: find domain and asymptotes and y intercept

hi Skylor,

I'm assuming that is

when x = 0

The vertical asymptotes are at x = -2 and x = + 3.

I'm never happy with these questions that ask for the domain.  I think the setter should be declaring a domain as part of the function definition rather than expecting the student to guess what domain is expected.

But, assuming the questioner means "The domain is the set of real numbers less any values that are inadmissible", then the inadmissible values are x = -2 and x = +3, because at these x values the f(x) value cannot be computed as it would involve division by zero.

As x tends to + or - infinity the graph approaches f(x) = 3x^2 so there are no horizontal asymptotes.

g(x) :  the quadratic can be similarly factorised to give the asymptotes and inadmissible values.

As x tends to + or - infinity the graph approaches g(x) = 2x.  As this is a line, it counts as an asymptote, but it is not horizontal.

Bob


You cannot teach a man anything;  you can only help him find it within himself..........Galileo Galilei

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#6 2012-10-22 04:29:18

skylor
Guest

Re: find domain and asymptotes and y intercept

I'm sorry I must have written the problem wrong

it should be f(x) = 3x^2+4 over x^2 -x -6. If you understand what I mean

#7 2012-10-22 06:01:28

bob bundy
Moderator
Registered: 2010-06-20
Posts: 6,531

Re: find domain and asymptotes and y intercept

Well no I don't.  I thought you meant

and that's the question I tried to answer in post 5.

If that isn't right then the only other interpretation I can think of is

That has x = 0 as an asymptote and doesn't cross the y axis at all.

This shows the importance of correct placing of brackets.

Oh wait a minute.  Did you mean:

In which case, same asymptotes as post 5 and same crossing .... in fact all the answers are unchanged, except .....

As x tends to infinity the function tends towards

So now we have a horizontal asymptote.

Bob


You cannot teach a man anything;  you can only help him find it within himself..........Galileo Galilei

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#8 2012-10-22 06:22:05

Skylor
Guest

Re: find domain and asymptotes and y intercept

Yes that it was I meant thanks!

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