Question 1142053
I think you meant to say "Show it is NOT a root."<br>

  {{{ pi }}} is a transcendental number - which means neither it, nor any rational multiple of it, is the root of ANY polynomial equation with integer coefficients.  
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It is the very definition of a transcendental number.

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EDIT: Oh, disregard, I just noticed cos in front, you should really write cos(2pi/9)
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cos(2pi/9) is approx 0.76604444

Plugging this into the equation gives 0 out, therefore cos(2pi/9) is a root.