$ x = rac\pi2 $: $ \cos 2x = \cos \pi = -1 $, $ \cos 4x = \cos 2\pi = 1 $, so $ f(x) = 1 - rac12(-1 - 1) = 1 - rac12(-2) = 1 + 1 = 2 $. Correct. - Nelissen Grade advocaten