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trouble solving diff. eqn. in alpha pro

Posted 10 years ago

I hope this is the correct forum for this question.

I typed in: x''+9x=sinx [also tried x''(t)+9x(t)=sin(x(t)), same result] into Wolfram Alpha but it timed out, so I finally relented and purchased the Wolfram Alpha Pro subscription on the Wolfram Alpha website. But the same input still would not return a solution, but continued to calculate for 30min. I'm pretty sure I've inputed diff. eqns. into wolfram alpha before and got a solution without having to buy Pro, but I can't swear to it.

Any help is appreciated.

Edit: nvm, lol, just realized I meant sint.

POSTED BY: danny tuttle

Try a numerical solution with two initial conditions to accommodate the second order. In Mathematica I was able to solve as follows.

Clear["Global`*"];
NDSolve[
 {x''[t] + 9 x[t] - Sin[x[t]] == 0, (*Two initial conditions.*)x[0] == 1, x'[0] == -1}, 
 x[t], {t, -2 Pi, 2 Pi}]
Plot[x[t] /. %, {t, -2 Pi, 2 Pi}]
POSTED BY: Isaac Abraham
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