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Old 01 June 2005, 06:52 PM
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matt85
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Default Scoobynet Maths bods.....

Can anybody differentiate these for me please...

1. y= 6/x^2 - 7/x^3 + 12x^3/4

2. y= (x^2 -6x - 2) 1nx

3. y= (x^4 - 8x^2 - 4) / (x^2 + 5x + 9)

4. y= (x^2 + 6x - 3)^8

...because im stuck!

Much obliged as ever.
Old 01 June 2005, 06:56 PM
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paul-s
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christ way over my head. looks like japanese to me
Old 01 June 2005, 07:14 PM
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Regulator
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Just a geuss, but can you use the rule of indicies to make the divisor the power?

e.g. 6/x² becomes 6^x-²
and then differentiate

Last edited by Regulator; 01 June 2005 at 07:39 PM.
Old 01 June 2005, 07:15 PM
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matt85
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Originally Posted by paul-s
christ way over my head. looks like japanese to me
Does to me as well!

Unfortunately i need the answer pretty sharpish otherwise im in the sh*t!

Anybody??
Old 01 June 2005, 07:20 PM
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john_s
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Eeeps! Haven't had to do that for ages & ages & ages....

John.
Old 01 June 2005, 07:24 PM
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Sprint Chief
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These don't look that hard? Although your notation is a bit confusing in places, so I might be misunderstanding the questions!

Number 1, I assume you mean 12x^(3/4)
Number 2, not sure what the 1nx means?
Old 01 June 2005, 07:26 PM
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Comper100
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Doing them all except 2. "y= (x^2 -6x - 2)1nx" Whats the 1nx bit at the end?

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Old 01 June 2005, 07:28 PM
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Sprint Chief
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Clue for number 1: apply the power rule, remember that 1/x^3 is the same as x^-3

Clue for number 3: apply the quotient rule

Number 4 could just be multiplied out.
Old 01 June 2005, 07:30 PM
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matt85
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Originally Posted by Sprint Chief
These don't look that hard? Although your notation is a bit confusing in places, so I might be misunderstanding the questions!

Number 1, I assume you mean 12x^(3/4)
Number 2, not sure what the 1nx means?
No.1. Yes, it should be 12x^(3/4), Schoolboy error

No.2. Honestly dont have a clue what it means either. Maybe it just goes as a constant, but i really dont know.

I just cant remember how to do them but i sure when i see the answers i will be able to work backwards and get my head around them.

Cheers.
Old 01 June 2005, 07:35 PM
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Sprint Chief
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Number 4 could be done by substitution and application of the chain rule, which would be quicker than multiplying out, but you could always just let Comper100 work it out for you

Not quite sure about Number 2 - if it was just times 1nx then why bother putting the 1 there? Confused!
Old 01 June 2005, 07:36 PM
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matt85
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Right.

So number 1 is:

1. y= 6/x^2 - 7/x^3 + 12x^3/4

y= 6x^-2 - 7x^-3 + 9x^(-1/4)

Correct??
Old 01 June 2005, 07:38 PM
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matt85
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Right.

So number 1 is:

1. y= 6/x^2 - 7/x^3 + 12x^3/4

y= 6x^-2 - 7x^-3 + 9x^(-1/4)

Correct??

EDIT- *correct so far?*
Old 01 June 2005, 07:39 PM
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is it "1nx" or "lnx"... as in the natural log of x?

John.
Old 01 June 2005, 07:40 PM
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Sprint Chief
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Not quite! 12x^(3/4) remains 12x^(3/4)
Old 01 June 2005, 07:41 PM
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Full marks to john_s! That makes a lot of sense! Then that one can be solved simply by the product rule.
Old 01 June 2005, 07:44 PM
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Sprint Chief
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Found this site which has nice graphical explanation:

Chain Rule
Quotient Rule
Product Rule
Old 01 June 2005, 07:45 PM
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matt85
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Aha, think ive cracked it (bear with me, please!).

Answer for no.1

y= (-12/x^3) - (-21/x^4) + (9/x^(1/4))


??
Old 01 June 2005, 07:46 PM
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Comper100
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For first on we get: dy/dx = -(12/x^3) + (21/x^4) + (9/X^(1/4)
Old 01 June 2005, 07:47 PM
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Comper100
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i agree
Old 01 June 2005, 07:50 PM
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matt85
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Originally Posted by Comper100
i agree
LOL! Good work
Old 01 June 2005, 07:53 PM
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If you ignore the 1nx then 2 is just dy/dx = 2x-6
Old 01 June 2005, 07:56 PM
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paul-s
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well ive learnt that if I EVER need to find out anything, someone on scoobynet will know the answer
Old 01 June 2005, 07:57 PM
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Oh and 3 with quoitent rule applied is: Hmm how am i going to write this?

dy/dx = ((x^2 - 5x - 9)(4x^3 - 16x) - (x^4 - 8x^2 - 4)(2x -5))/((x^2 - 5x - 9)^2))

Hows that
Old 01 June 2005, 07:59 PM
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No. 4 i think is

8 (x^2+6x-3)^7 (2x+6)
Old 01 June 2005, 08:02 PM
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Wouldn't the first part of 3 be

(4x^3 - 16x)/(x^2 + 5x + 9)

and the denonimator of the 2nd part

(x^2 + 5x + 9)^2

ie
(4x^3 - 16x)/(x^2 + 5x + 9) - ((x^4 - 8x^2 - 4)(2x -5))/((x^2 + 5x + 9)^2)
?

Last edited by krazy; 01 June 2005 at 08:14 PM.
Old 01 June 2005, 08:10 PM
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krazy
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Assuming 2 is Lnx then i's guess

(x^2 - 6x -2)/x - (2x-6)Lnx




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