Show that (1 + 2cos) is a factor of f() = 8cos3 - 4cos – 1\t

Dear student,

I will denote theta by x (for simpliciy)

so , we can write f(x) as
f(x) = (8cos³x + 1) – 4 cosx – 2
= (2cosx + 1) (4cos²x + 1 – 2cosx – 2)

Thanks.