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

cos⁡xcos⁡6x=−1 or  2cos⁡xcos⁡6x=−2 or  cos⁡7x+cos⁡5x=−2 or  cos⁡7x=−1 and cos⁡5x=−1The value of x satisfying these two equations simultaneously and lying between 0 and 2π isπ. Therefore, the general solution is x=2nπ+π,n∈Zthus, x=(2n+1)π,n∈Z