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- March 1st 2008, 03:06 AM #1

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## What to do?

Hi

i got this question and not sure what its about:

Find the value of x that minimises

y = 8X^2 (400/X)

for positive x.

Give the required x both exactly (i.e. as a surd).

What do i do?

This is what i started doing but no idea what else:

=> y=8x^2+(400/x)

=> 16x-400x^(-2)

==> 16x-400x^(-2) = 0

Thanks

- March 1st 2008, 03:30 AM #2

- March 1st 2008, 03:33 AM #3

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- March 1st 2008, 03:43 AM #4

- March 1st 2008, 03:46 AM #5

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- March 1st 2008, 03:53 AM #6
Wrong for several reasons, including the fact that there's no = sign and therefore you have not given an equation.

16x-400x^(-2) = 0 multiplied through by x^2 gives 16x^3 - 400 = 0.

Do you understand that x^(-2) times x^2 = x^0 = 1? You need to revise your basic algebraic skills.

- March 1st 2008, 04:01 AM #7

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- March 1st 2008, 05:47 AM #8