Math Is Fun Forum

AISSAISHAK

Guest

Weird question in Math Exam, NEED HELP PLZ!!?

f(x)=x(1+[1/sqrt(x^2+1)])=x+[x/sqrt(x^2+1)]

they already gave us the answer... f ' (x)= 1+[1/(x^(2)+1)(sqrt(x^(2)+1))

here is the equation http://oi54.tinypic.com/vwz97o.jpg

but they want us to find the derivative in details.

for me I couldnt find the right answer. could u please help with that...
thank you in advance.

AISSAISHAK2

Guest

Re: Weird question in Math Exam, NEED HELP PLZ!!?

I'm sry, but what hhappeend to the answer?!

bobbym

bumpkin

From: Bumpkinland

Registered: 2009-04-12

Posts: 109,606

Re: Weird question in Math Exam, NEED HELP PLZ!!?

Hi;

This is long but it uses only simple substitutions and the chain rule and product rule.

I am going to leave out the x in front because it is just a 1 and we will add later.

Use the product rule:

We need the derivative of the boxed one.

Use the substitutions:

With the chain rule:

That is the last piece of the product in A)

Clean up a bit:

Add the one!

Let's try make it look like your answer:

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In mathematics, you don't understand things. You just get used to them.
If it ain't broke, fix it until it is.
Always satisfy the Prime Directive of getting the right answer above all else.

AISSAISHAK3

Guest

Re: Weird question in Math Exam, NEED HELP PLZ!!?

hello bobbym, thank u so much for giving ur time on this

on the picture,, is that correct, coz i'm having some thoughts that there is an error!

coz the question tells that we must find f'(x) or dy/dx equals this 1+( 1 / ( ((x^2)+1)*sqroot( (x^2)+1 ) >>> just like in the picture

and we must write in steps until we find that answer above

so if you understand it, plz give me the right answer!

thank you again!

bobbym

bumpkin

From: Bumpkinland

Registered: 2009-04-12

Posts: 109,606

Re: Weird question in Math Exam, NEED HELP PLZ!!?

Hi AISSAISHAK3;

Welcome to the forum! Sorry it took much longer than it should have because I got stuck on 2 dead ends that looked good but ballooned out. There was a nice trig substitution but I made so many errors that I went back to the long way.

Here is another way: I leave the x again because it is a 1 when differentiated. I will add it at the end.

Use the natural sub: x = tan(u):

Use the chain rule:

You have a composition of functions to work with. Go to this page:

http://staff.jccc.net/swilson/trig/compositions.htm

You will see that

So:

Do not forget to add the one:

I suggest you use the one in post #3.

---

In mathematics, you don't understand things. You just get used to them.
If it ain't broke, fix it until it is.
Always satisfy the Prime Directive of getting the right answer above all else.

soroban

Member

Registered: 2007-03-09

Posts: 452

Re: Weird question in Math Exam, NEED HELP PLZ!!?

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AISSAISHAK4

Guest

Re: Weird question in Math Exam, NEED HELP PLZ!!?

thank you alll for the replies, that helped me alot.. it tookk me an hour to solve that but without luck... I guess I should learn more about this tricks.

thank you again

ah, something else, I cant get my self registered I never received any emails about my account .. is there an other way to register?

unEx

Guest

Re: Weird question in Math Exam, NEED HELP PLZ!!?

hello,
I found a really easy and short one
check it out!

Bob

Administrator

Registered: 2010-06-20

Posts: 10,649

Re: Weird question in Math Exam, NEED HELP PLZ!!?

hi AISSAISHAK

Your posts show you as a guest not a member.

At the top of the page there is a register button.  Click this, agree to the rules, and fill in your details.

You will get emails if you 'subscribe' to a post thread.

Hope that helps, welcome to the forum,

Bob

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Children are not defined by school ...........The Fonz
You cannot teach a man anything;  you can only help him find it within himself..........Galileo Galilei
Sometimes I deliberately make mistakes, just to test you!  …………….Bob

bobbym

bumpkin

From: Bumpkinland

Registered: 2009-04-12

Posts: 109,606

Re: Weird question in Math Exam, NEED HELP PLZ!!?

Hi AISSAISHAK4;

That is why I heartily recommend post #3. It is the one that uses the least tricks or specialized rules. Did you register yet? You will need an active email address.

I never received any emails about my account

Have you checked to see that the email address you provided can receive emails? Perhaps the email was filtered out to the spam or junk folder. Some email services are too aggressive with their filtering.

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In mathematics, you don't understand things. You just get used to them.
If it ain't broke, fix it until it is.
Always satisfy the Prime Directive of getting the right answer above all else.