[SOLVED] Differential Equations

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April 28th 2008, 12:08 PM
eggy524

[SOLVED] Differential Equations

Hi there, I have a question on differential equations I dont understand, here it is:

find the general solution:

t (dx/dt) = x + (sin(x/t))^2

I think you have to use substitution for this one but dont know how, any ideas?
April 28th 2008, 01:42 PM
TwistedOne151

Define $u=\frac{x}{t}$ . Then $\frac{du}{dt}=-\frac{x}{t^2}+\frac{1}{t}\frac{dx}{dt}$ . Now, our original equation tells us $\frac{dx}{dt}=\frac{x}{t}+\frac{1}{t}\sin^2\frac{x }{t}$ , and so:
$\frac{du}{dt}=-\frac{x}{t^2}+\frac{1}{t}\left(\frac{x}{t}+\frac{1 }{t}\sin^2\frac{x}{t}\right)$
$\frac{du}{dt}=\frac{1}{t^2}\sin^2\frac{x}{t}$
$\frac{du}{dt}=\frac{\sin^2{u}}{t^2}$
Which you should be able to solve for u(t). Then just use x=ut to find x.

--Kevin C.

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