Reducible Second Order Differential Equation: Ind. and First Derivative Missing

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SUMMARY

The discussion focuses on solving the reducible second-order differential equation given by y^3 * y'' = 1. The user employs the substitution p = dy/dx and transforms the equation into a separable form, leading to the integral (1/2)p^2 = -(1/2)y^-2 + C. The solution process involves integrating to find p and subsequently dx/dy. The user realizes the need to correctly handle the integration process, particularly in relation to the derivative, which is crucial for arriving at the final solution.

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  • Understanding of second-order differential equations
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  • Knowledge of integration techniques, including handling square roots in integrals
  • Basic calculus concepts, particularly derivatives and integrals
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JTemple
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Homework Statement



Solve the reducible 2ODE. Assume x, y and/or y' positive where helpful.

y^3 * y'' = 1


The Attempt at a Solution



Well, I tried what I normally would do for x being missing.

p = (dy/dx); y'' = p'p = (dp/dy)(dy/dx)

So

y^3 p'p = 1

p(dp/dy) = y^(-3)

Separate, Integrate

\intpdp= \inty^-3dy

(1/2)p^2 = -(1/2)y^-2 + C

p = (2C - (y^-2))^(1/2)

1/p = dx/dy, so

dx = (2C - (y^-2))^(1/2) dy

And then I would try and integrate that. I can't get the right side (I thought arcsin, but aren't I missing the du (Calc II memory bad), or do I add 0 in the form of + something - something in the numerator? Or is my overall approach off?
 
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You have

p=\sqrt{2\,C-\frac{1}{y^2}}\Rightarrow \frac{d\,y}{d\,x}=\frac{\sqrt{2\,C\,y^2-1}}{y}\Rightarrow \int\frac{y\,d\,y}{\sqrt{2\,C\,y^2-1}}=\int d\,x

Let
y^2=\frac{z}{2\,C}
and the 1st integral can be easily computed.
 
That's where it was, I was missing the part of the derivative for integration. Thanks!
 

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