Problems fitting

Hello ROOTers!!

I am new here and I really don’t know if I am having problems with ROOT.
I was studying the exponential decay of a damped pendulum from my physics class, and I am supposed to fit the data, but I got this weird thing here :

Am I doing some mistake?

//using TGraph *pontos = new TGraph(n, px, py);
  	TF1 *theta = new TF1("Funcao theta", "0.157*exp([0]*x)*cos([1]*x+[2])", 0, 30); //0.157 was measured

Damped Pendulum:

Already solved, sorry for the inconvenience :blush:

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