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HELP PLEASE stationary values


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#1 AM4R

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Posted 08 November 2006 - 08:04 PM

Can someone please please try and help me with this question thanks.

Find the st values and determine each stationary value (min or max)

y=e^-x(sinx)

When i differentiate I get -e^-x = 0.

What do I do when I get to this stage?? blink.gif

Thanks
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#2 Discogirl17

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Posted 08 November 2006 - 08:20 PM

Use logs of base e on both sides and u'll end up with x= log of base e to the 0. I think!
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#3 AM4R

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Posted 08 November 2006 - 08:30 PM

smile.gif Thanks for the help but how do I do that?
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#4 Steve

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Posted 08 November 2006 - 09:03 PM

That's a good question! You can't - think about the graph of y=e^x, this is never zero.

You've not differentiated correctly, I get \frac{dy}{dx}=e^{-x}(\cos x-\sin x).

Does this help?
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#5 AM4R

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Posted 08 November 2006 - 09:57 PM

yup your right, but where do I go from there? and how do I factorise it?

Thanks
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#6 Discogirl17

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Posted 09 November 2006 - 11:17 AM

Ah yes he's right. You must now use logs of base e. So you get log to the base e of e to the power of -x, this simply becomes -x since log of e to the e is 1. So you have -x times (cos x- sinx) which becomes -xcosx+xsinx. I'm sure you (using the standard formulas given in past papers and textbooks etc)can rearrange this to form the answer.

P.S I may be wrong, maybe its best to PM Dave, he's the maths wizard! Maths isn't exactly my forte!
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#7 Steve

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Posted 09 November 2006 - 07:36 PM

Sorry Discogirl, but that's completely wrong!

We want to solve



\begin{align*}\frac{dy}{dx}&=0\\
e^{-x}(\cos x-\sin x)&=0\end{align*}

So, just in the usual way, e^{-x}=0 or \cos x-\sin x=0. The first equation has no solutions, but it remains to find solutions to the second one, i.e. \cos x=\sin x.

This is something you'll have done at Higher.
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#8 Discogirl17

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Posted 11 November 2006 - 10:30 PM

Like I said, maths isnt my forte lol!
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#9 Dave

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Posted 11 November 2006 - 11:10 PM

QUOTE(Discogirl17 @ Nov 9 2006, 11:17 AM) View Post


P.S I may be wrong, maybe its best to PM Dave, he's the maths wizard! Maths isn't exactly my forte!



lol i have her well trained

If i am not here i am somewhere else







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