But earlier we said $ f(x) = 1 - rac12(\cos 2x - \cos 4x) $. When $ \cos 2x = -1 $, $ \cos 4x = 1 $, so $ -1 - 1 = -2 $, multiplied by $ -rac12 $ gives $ +1 $, so $ 1 + 1 = 2 $. Yes. - go-checkin.com