The general solution of `cosxcos6x=-1` is `x=(2n+1)pi,n in Z` `x=2npi,n in Z` `x=npi,n in Z` (d) none of these

Explanation

`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`


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