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February 16th, 2016, 04:24 PM   #1
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Please Help with d2y/dx2

What is d2y/dx2 of y^4=x^5?

I have tried everything, every calculator or website gives me a different answer.

Please help, thanks.
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February 16th, 2016, 04:33 PM   #2
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$\displaystyle y=x^{5/4},\quad x\ge0$

Can you continue?
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February 16th, 2016, 04:34 PM   #3
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Can you give at least one of your attempts, showing your working?
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February 16th, 2016, 04:59 PM   #4
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y^4=x^5
4y^3y'=5x^4
y'=(5x^4)/(4y^3)=(5x^4*xy)/(4y^3*xy)=5y/4x * x^5/y^4 = 5y/4x

y'' of 5y/4x=(4x(5y')-5y(4))/12x^2
= (4x(5-(5y/4x))-20y)/12x^2
=15y/12x^2
= 15x^3/12y^2

This is the last exercise of my webassign, I am trying to solve since 12.30pm and cant find the answer. Have tried multiple ways according to the sites i visited but all answer are incorrect. Dont really know what i am doing wrong.
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February 16th, 2016, 05:35 PM   #5
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Got it! (5x^3)/(16y^3)
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February 16th, 2016, 05:57 PM   #6
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You frequently omitted required parentheses to enclose the denominator.
You gave 12x^2 instead of (16x^2) in your third and subsequent lines.
In the next line, the first "-" shouldn't be present.
In the next line, 15 should be 5.
Hence you should have obtained 5y/(16x^2),
which equals (5x^3)/(16y^3) (but this last step isn't needed).
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