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# C4 - Differentiating exponential equations watch

1. x = 15 - 12e^-t/14

show that dx/dt = 1/14(15-x)

im so stuck where the 15-x comes from...

someone help out pls
2. (Original post by Gary)
x = 15 - 12e^-t/14

show that dx/dt = 1/4(15-x)

im so stuck where the 15-x comes from...

someone help out pls
It comes from the original equation for x.

3. (Original post by Gary)
x = 15 - 12e^-t/14

show that dx/dt = 1/4(15-x)

im so stuck where the 15-x comes from...

someone help out pls
What's 15-15?
4. There is an error in the question by the way but I presume it is a typo ...?
5. The question is on AQA June 2007, question 4ci) if you want a better look at it

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6. (Original post by Gary)
The question is on AQA June 2007 / 4ci) if you want a better look at it
Can't say that I do.
7. (Original post by Mr M)
Can't say that I do.
Well I don't think there's an error in the question as I checked and there's no typo

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8. (Original post by Gary)
Well I don't think there's an error in the question as I checked and there's no typo

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Yes there is.
9. Oh yeah... Should be dy/dx = 1/14(15-x)

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10. (Original post by Gary)
Oh yeah... Should be dy/dx = 1/14(15-x)

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So have you solved it yet or are you still struggling?
11. Nahh I'm stuck..

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12. (Original post by Gary)
Nahh I'm stuck..

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Differentiate the equation for x to find dx/dt.

Then replace the bit by changing the subject of the equation for x.
13. So I get dx/dt = 6/7e^-t/14

But I don't get how you replace it by changing the subject of the equation for x

Is it possible you post the soltuion step by step?

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14. x= 15 -12e^(t/14)
dx/dt= -12e^(t/14) * -1/14
Rearrange origininal eq to get (15-x)=12e^(t/14)
Sub in
dx/dt=-(15-x)*-1/14
=1/14(15-x)

Pretty much what guy above said. Hope it helps.
15. So it was that simply all along.... Sigh

Thank you very much for the help!

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