First step of this simple "limit" problem


by sachin_naik04
Tags: limit, simple, step
sachin_naik04
sachin_naik04 is offline
#1
Dec28-09, 12:14 PM
P: 13
Lim.............X^3-3x^2+4
x->2........... X^4-8x^2+16

In the above "limit" problem i would like to know only the very first important step, the later part of the problem i will manage to solve as its easy, only the first step is a bit tricky

so what could be the first step of this problem

actually i am a B.com. student, plz help me thanks in advance
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matticus
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#2
Dec28-09, 12:26 PM
P: 107
l'hospital's rule applies nicely. if you haven't learned that, then just factor the numerator and denominator.
sachin_naik04
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#3
Dec28-09, 10:30 PM
P: 13
no i haven't learned any l'hospitals rule, but how do i factor the denominator as its x^4

i want to solve it without that l'hospital method, plz help

oh i am hearing that l'hospitals rule is easy, i hope i will understand, so if possible u can also explain this rule to solve the first step of this problem or the other
but atleast something plz

Mark44
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#4
Dec28-09, 10:40 PM
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First step of this simple "limit" problem


The denominator factors into (x2 - 4)2, which can be further factored into (x - 2)2(x + 2)2. The numerator can also be factored. Some obvious candidates are (x - 1), (x + 1), (x - 2), (x + 2), (x - 4), and (x + 4).
sachin_naik04
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#5
Dec28-09, 10:47 PM
P: 13
@Mark44

oh thanks a lot for your help really great

i will try it on my book and see if i have any difficulty, if yes then i will come back here
sachin_naik04
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#6
Dec28-09, 10:50 PM
P: 13
but one more thing in order to get that (x^2 - 4)^2 have u used your own mind or have u done some rough work
HallsofIvy
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#7
Dec29-09, 05:24 AM
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The fact that Mark44 wrote that in terms of [itex]x^2- 4[/itex] rather than x- 2 implies that he first recognized that [itex]x^4- 8x^2+16= (x^2)^2- 2(4)x^2+ 4^2[/itex] is of the form "[itex]u^2- 2au+ a^2[/itex]", the standard form of a perfect square. I would have done this in a completely different way:

The only reason you had a problem with that fraction was because both numerator and denominator go to 0 as x goes to 2. That tells you immediately that each polynomial has x- 2 as a factor. Divide the polynomial by x- 2 to find the other factor. Repeat if necessary.
DarkOni
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#8
Dec29-09, 06:43 AM
P: 4
I think u should try learning L'Hospital's rule too.....all you have to do then is differentiate the numerator & denominator separately(only in cases: 0/0, ∞/∞, etc.) and then apply the limit.
Mentallic
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#9
Dec29-09, 08:48 AM
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Actually I would've gone with a combination of both Mark44's and Hallsofivy's ideas.

Firstly, looking at the denominator, yes it's a quartic ([itex]x^4[/itex]) but you can treat it as a quadratic in some other variable.

[tex]x^4-8x^2+16[/tex]

if you let [itex]x^2=u[/itex] you then have:

[tex]u^2-8u+16[/tex]

which is a quadratic. Factoring this gives you:

[tex](u-4)^2[/tex]

And then you can substitute [itex]u=x^2[/itex] back and factorize further as Mark44 has done.

For the numerator it is a cubic, so the above method cannot be used but you can then factor out (x-2) since when you substitute x=2 into it, you end up with 0. This of course means x=2 is a root of the equation.
After that you'll have a quadratic so it's straight-forward from there
sachin_naik04
sachin_naik04 is offline
#10
Dec29-09, 09:23 AM
P: 13
well i understood the problem,thanks everyone a lot

now i have one more problem for the following simple limit


lim.........3x(x2-7x+6)
x->3......(x+2)(x3+4x+3)


now if u see the above problem and substitute the value of "x" that is "3" then u get the answer as -9/35 therefore the denominator doesn't go to "0" and hence the answer but the text book says the correct answer for this problem is 5/2, looks like we have to simplify the problem is it so, because i had heard that whenever the denominator does NOT go to "0" then we can directly substitute the value of x without simplifying the problem

so the question is should i directly substitute the value of "x", as the denominator does not go to "0" or i have to simplify it, if yes then why?
Mark44
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#11
Dec29-09, 11:30 AM
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Since the denominator doesn't approach 0 as x approaches 3, you can simply substitute 3 in both the numerator and denominator. The value I get is also -9/35. If your book gets a different value, make sure that you are working the same problem as in your book.
sachin_naik04
sachin_naik04 is offline
#12
Dec30-09, 08:03 AM
P: 13
ya thanks a lot


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