- #1
_N3WTON_
- 351
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Homework Statement
Solve the given differential equations with missing x.
[itex] y'' + y = 0 [/itex]
Homework Equations
[itex] y = c_1cos(x) + c_2sin(x) [/itex]
This is the answer given in the back of the book. However, I can't sem to get my answer to agree
The Attempt at a Solution
First, I made some substitutions:
[itex] y' = v y'' = v' [/itex]
Using the chain rule I obtained:
[itex] \frac{dv}{dx} = v\frac{dv}{dy} = v' [/itex]
So the equation becomes:
[itex] v\frac{dv}{dy} + y = 0 [/itex]
[itex] vdv = -ydy [/itex]
[itex] \int vdv = -\int ydy [/itex]
[itex] \frac{v^2}{2} = - \frac{y^2}{2} + C_1 [/itex]
[itex] v = +- sqrt(-y + C_1) [/itex]
[itex] \frac{dy}{dx} = sqrt(-y + C_1) [/itex]
[itex] \int\frac{dy}{dx} = \int sqrt(-y+C_1),dx [/itex]
[itex] y = \frac{2}{3} (-y+C_1)^{\frac{3}{2}} + C_2 [/itex]
Clearly this solution is nowhere close to the one in the back of the book, so I was hoping somebody could point out where I have gone wrong :)