`cosxcos6x = -1`
`=>cos6x = -1/cosx`
`=>cos6x = -secx`
Now, we know, `cos x in [-1,1]` and `sec x in [-oo,-1] uu [1,oo]`
So, only two below cases are possible for the given solution.
Case 1: When `cosx = -1 and cos6x = 1`
`=>x = (2n+1)pi`
Case 2: When `cosx = 1 and cos6x = -1`
It is not possible as when `cos x = 1` , then `cos6x ` can not be `-1`.
So, only one solution possible, that is `x = (2n+1)pi, n in Z`
