Let α,β be any two positive value of x for which 2cosx, |cosx| and 1 - 3cos2x are in G.P. The minimum value of |α−β| is
cos2x=2cosx(1−3cos2x)
cosx {6cos2x+cosx−2}=0
cosx = 0,12,−23
Thus smallest value of x are π3 and π2
⇒|α−β|=π6