Show that cosπ/7,cos3π/7,cos5π/7 are roots of the equation 8x^3-4x^2-4x+1=0 and deduce the equation

B.Sc. Mathematics:Trigonometry:Deduction from De Moivre's Theorem:Show that cosπ/7,cos3π/7,cos5π/7 are roots of the equation 8x^3-4x^2-4x+1=0 and deduce the equation whose roots are tan^2(π/7),tan^2(3π/7),tan^2(5π/7)