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## Homework Statement

Find the equations of the trajectories of y"+y^3=0.

## Homework Equations

None.

## The Attempt at a Solution

The Undamped Case:

y"+p(y)=0

v(dv/dy)+p(y)=0

integrate

v^2/2+P(y)=C

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So following the formula above:

The answer I got is v^2/2+y^4/4=C, but the answer in the book says v^2+y^4/4=C. What's wrong?