If x∈ (3π4,π ), then the value of cosx+cos 2x(2cosx+1)(cosx-1) is
cosx+cos 2x(2cosx+1)(cosx-1) =-cosx+cos2x(2cosx+1)(cosx-1) ( cosx=-cosx when x∈ (3π4,π) and cos2x= cos2x when 2x∈ (3π2,2π))⇒ given question =-cosx+2cos2x-1(2cosx+1)(cosx-1)
=2 cosx +1cosx-12cosx +1cosx-1=1