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# How the hell to make x the subject? watch

1. *pic below*

could someone please tell me how to make x the subject? I've been trying for ages. Trying to make dA/dx = 0 so only the top part = 0 and just stuck
2. any help is appreciated
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3. Is that the whole question?
4. (Original post by zezno)
*pic below*

could someone please tell me how to make x the subject? I've been trying for ages. Trying to make dA/dx = 0 so only the top part = 0 and just stuck
(Original post by zezno)
any help is appreciated
What's the actual question?? Sure you can make the differential equal 0 to find the points where is minimised/maximised.

In which case you have

and note that then carry on from there.
5. (Original post by RDKGames)
What's the actual question?? Sure you can make the differential equal 0 to find the points where is minimised/maximised.

In which case you have

and note that then carry on from there.

I always get stuck when there's more than 1 x term with powers involved.

So I noticed that you can cancel out the (x^2 -50) which gave me:

-x(100-x^2)^1/2 = 48

I tried getting rid of the square root so now:

-x(100-x^2) = 2304

expanding

-100x + x^3 = 2304

and yeah stuck lol two x's, factorising would give me though:

x(-100+x^2) = 2304
6. (Original post by zezno)
I always get stuck when there's more than 1 x term with powers involved.
...

I tried getting rid of the square root so now:

-x(100-x^2) = 2304

...
This should be
7. (Original post by RDKGames)
This should be
ah okay. Then what next?
8. (Original post by zezno)
ah okay. Then what next?
Achieve the form then you have a quartic which you can treat as a quadratic with a simple substitution and solve for by solving the quadratic then solve for itself.

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Updated: February 3, 2017
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