let x = 2cos(π/5)

use complex numbers to show that

x^4 - 3x^2 + 1 = 0

I'm stumped completely

I tried substituting the value in and using trig identities.. got a big mess. I tried treating the x as the x part of a complx number... still more mess.

Anything to at least lead me in the right direction... I'm sure it's staring me in the face.