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I need to solve the following differential equation

eqn = y''[t] - a *Cos[x[t]]*x''[t] - a *Sin[x[t]]*(x'[t])^2 == 0;

with initial conditions y[0]=0 and y'[0]=0. And y[t]=a Sin[x[t]] is the connection between the two variables x and y.

Help please.

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  • $\begingroup$ What issues with DSolve you have? It's pretty simple. $\endgroup$
    – m0nhawk
    May 22, 2017 at 17:45
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    $\begingroup$ You have two dependent variables, x and y. You need two equations. $\endgroup$
    – Bob Hanlon
    May 22, 2017 at 17:45
  • $\begingroup$ @Bob Hanlon. You are right. I have just added that equation to my question. Could you kindly suggest how to solve this system? $\endgroup$
    – Jee
    May 22, 2017 at 18:00
  • $\begingroup$ Oh yes. Sorry for the poor notation. x is the function of time. Thanks @David $\endgroup$
    – Jee
    May 22, 2017 at 18:16
  • $\begingroup$ So you eqn is eqn = y''[t] - a*Cos[x[t]]*x''[t] - a*Sin[x[t]]*(x'[t])^2 == 0 /. y -> Function[t, a Sin[x[t]]] // FullSimplify ? $\endgroup$ May 22, 2017 at 18:19

1 Answer 1

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Your equation only has a constant solution. You can see this by replacing for y:

eqn /. y -> ( a Sin[x[#]] &)
(*result: -2 a Sin[x[t]] x''[t]^2 == 0*)

So your differential equation actually is $\sin(x) \left(\frac{\partial^2x}{\partial t^2}\right)^2=0$. The only way to satisfy this equation is $x=const$.

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