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quicksilver123
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Homework Statement
Find y''
Homework Equations
9x^2 +y^2 = 9
The Attempt at a Solution
y'
18x+2y(y')=0
y'=-18x/2y
y'=9x/y
For the second derivative, I get the correct answer (same as the book) up until the very last step.
Here's where I'm left at:
-9( (-9x^2 - y^2) / y^3 )
The book then takes this answer to:
y''= -9 (9/y^3) [since x and y must satisfy the original equation, 9x^2+y^2=9] Thus y''= - 81/y^3
I'm not sure how they got here as my final answer ends up something like -81x^2/y^3 -9y (probably with some sign mistakes in there but I was too distracted with the book's justification for their answer to worry about that).
Help?
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