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The general solution of cos x cos 6x =-1...

The general solution of `cos x cos 6x =-1` is

A

`x=(2n+1) pi, n in Z`

B

`x=2n pi, n in Z`

C

`x= n pi, n in Z`

D

none of these

Text Solution

Verified by Experts

The correct Answer is:
A
`cos x cos 6x=-1`
or `2 cos x cos 6x=-2`
or `cos 7x+ cos 5x=-2`
or `cos 7x =-1 and cos 5x=-1`
The value of x satisfying these two equations simultaneously and lying between 0 and `2pi` is `pi`.
Therefore, the general solution is `x=2npi+pi, n in Z`. Thus,
`x=(2n+1) pi, n in Z`
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