>> theta=-pi:0.01:pi;
>> y=f(theta);
>> plot(theta,y);
>> grid

>> format long
>> fzero(@f,-.7)

ans =

  -0.720849204460389

>> fzero(@f,-.3)

ans =

  -0.331005184283869

>> fzero(@f,1.2)

ans =

   1.143685517821374

>> fzero(@f,2.3)

ans =

   2.115909014086458
f