Phasor notation of a complex number with apparently non-constant radius
Hint: a phasor is defined by its magnitude and phase. The magnitude can be obtained by computing

$$ 
    \sqrt{\left(\mathcal{Re}\left\{x_1[n]\right\}\right)^2 
      + \left(\mathcal{Im}\left\{x_1[n]\right\}\right)^2}
$$

And the phase can be obtained with

$$
    \tan^{-1}{\frac{\mathcal{Im}\left\{x_1[n]\right\}}{\mathcal{Re}\left\{x_1[n]\right\}}}
$$