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From: Virgil on 5 Apr 2010 04:00
In article
<9d5c4a3a-e9d5-41d7-bca5-4013ac193640(a)r1g2000yqb.googlegroups.com>,
Sanny <softtanks22(a)hotmail.com> wrote:

> > > Here is the Question.
> >
> > > lim (x-> 2) [x^n - 2^n] / [x-2] == 80
> >
> > > In the above equation what is the value of "n"
> >
> > > Solve it and tell me?
> >
> > This problem is trivial; you should be able to solve it yourself,
> > especially if you are 1/10 as good think you are.
>
> The girl told me a formula using that it can be solved in 5 sec.
>
> But I am not convinced with that formula.
>
> 1. From where that formula came?

Do you mean the formula for dividing x^n - y^n by x - y ?
>
> 2. That formula is very generalized only to solve this question. Its
> like memorizing all multiplication tables instead of using a
> calculator.

That is a formula that appears quite frequently in problems involving
algebraic manipulation, and is hardly limited to just this problem.
>
> I want it to be solved without use of any formula. But be solved as an
> original problem.
>
> Can you solve it without using any artificial formula.

Do you consider l'Hopital's rule artificial?
>
> Incase there is a formula can you derive that formula?
>
> Bye
> Sanny
>
> Ask your Physics Question:
> http://www.getclub.com/Problems.php?cat=Physics
>
>
> Or Earn money Solving others Questions.
> http://www.getclub.com/Problems.php?cat=Physics
From: Helmut Wabnig hwabnig on 5 Apr 2010 04:04
On Mon, 5 Apr 2010 00:53:09 -0700 (PDT), Sanny
<softtanks22(a)hotmail.com> wrote:

>> n = 5
>
>I knew the answer is 5 and the formula used.
>

So you are playing us up.
You did not give all relevant information in your original posting
and we had to worm it out of you.

Don't like that too much.

Ok, then let us know what you mean by "the formula used".

>I am asking how that formula came?
>
>Please solve it in steps.
>
>I had left Mathematics 20 years back and I have never read a Math book
>for last 20 years So dont accept me to remember all formulas.
>
>

There is no "formula" to remember, but procedures.
You do know how to differentiate an expression like

y= (x^n - 2^n)

into

dy/dx = ...?


w.
From: Sanny on 5 Apr 2010 04:11
> > Can you solve it without using any artificial formula.
>
> Do you consider l'Hopital's rule artificial?

Ok, Now I understand It used L'Hôpital's rule.

The girl just gave me the equation without telling that it used
L'Hôpital's rule.

> > Incase there is a formula can you derive that formula?

I have seen how L'Hôpital's rule is prooved.

Bye
Sanny

Ask your Physics Question:
http://www.getclub.com/Problems.php?cat=Physics

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From: Dirk Van de moortel on 5 Apr 2010 04:13
Virgil <Virgil(a)home.esc> wrote in message
Virgil-8279EA.02002805042010(a)bignews.usenetmonster.com
> In article
> <9d5c4a3a-e9d5-41d7-bca5-4013ac193640(a)r1g2000yqb.googlegroups.com>,
> Sanny <softtanks22(a)hotmail.com> wrote:
>
>>>> Here is the Question.
>>>
>>>> lim (x-> 2) [x^n - 2^n] / [x-2] == 80
>>>
>>>> In the above equation what is the value of "n"
>>>
>>>> Solve it and tell me?
>>>
>>> This problem is trivial; you should be able to solve it yourself,
>>> especially if you are 1/10 as good think you are.
>>
>> The girl told me a formula using that it can be solved in 5 sec.
>>
>> But I am not convinced with that formula.
>>
>> 1. From where that formula came?
>
> Do you mean the formula for dividing x^n - y^n by x - y ?
>>
>> 2. That formula is very generalized only to solve this question. Its
>> like memorizing all multiplication tables instead of using a
>> calculator.
>
> That is a formula that appears quite frequently in problems involving
> algebraic manipulation, and is hardly limited to just this problem.
>>
>> I want it to be solved without use of any formula. But be solved as an
>> original problem.
>>
>> Can you solve it without using any artificial formula.
>
> Do you consider l'Hopital's rule artificial?


No need for that.
You didn't recognize the definition of the derivative in there?

Just note that he's looking for df/dx(2) where f(x) = x^n, so
df/dx = n x^(n-1),
so he's looking to find an integer solution for
n 2^(n-1) = 80

Trial and error.

Dirk Vdm
From: Sanny on 5 Apr 2010 04:14
> >I had left Mathematics 20 years back and I have never read a Math book
> >for last 20 years So dont accept me to remember all formulas.
>
> There is no "formula" to remember, but procedures.
> You do know how to differentiate an expression like

I am satisfied with "Virgil" response. He told it needs L'Hôpital's
rule.

Actually the girl just asked me to use this equation

nx^n-1=80

So I was confused from where it came. Incase she had told me that she
used L'Hôpital's rule then I would have understood it quickly.

So I know its just a simple differentiation.

Bye
Sanny

Ask your Physics Question:
http://www.getclub.com/Problems.php?cat=Physics

Or Earn money Solving others Questions.
http://www.getclub.com/Problems.php?cat=Physics
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