Prove that `4cos(2pi)/7dotcospi/7-1=2cos(2pi)/7dot`

Prove that 4cos((2pi)/7).cos(pi/7)-1=2cos((2pi)/7) .

prove that 4(cos(2 pi))/(7)*(cos pi)/(7)-1=2(cos(2 pi))/(7)

Prove that cos((2pi)/7) cos((4pi)/7) cos((8pi)/7) =1/8

Prove that cos((2pi)/7)cos((4pi)/7)cos((8pi)/7)=1/8

Prove that: cos(pi)/(7)cos(2 pi)/(7)cos(4 pi)/(7)=-(1)/(8)

prove that cos2(pi)/(7)+cos4(pi)/(7)+cos6(pi)/(7)=-(1)/(2)

Prove that :(cos pi)/(7)(cos(2 pi))/(7)(cos(3 pi))/(7)(cos(4 pi))/(7)=(1)/(8)

cos.(2pi)/(7)+cos.(4pi)/(7)+cos.(6pi)/(7)

Using application of trignometric formulas prove that (i)cos(pi/4+x)+cos(pi/4-x)=sqrt2cos x(i1)sin(7pi/12)cos(pi/4)-cos(7pi/12)sin(pi/4)