Rút gọn biểu thức $$M = \sin x + \sin \left( {x + \frac{{16\pi }}{5}} \right) + \sin \left( {x + \frac{{22\pi }}{5}} \right) + \sin \left( {x + \frac{{28\pi }}{5}} \right) + \sin \left( {x + \frac{{34\pi }}{5}} \right)$$