If 2 cos theta = x + 1/x, prove that 2 cos 3 theta = x cube + 1/x cube. Share with your friends Share 14 Manbar Singh answered this We have, 2 cos θ = x + 1xcubing both sides, we get 2 cos θ3 = x + 1x3⇒8 cos3θ = x3 + 1x3 + 3 × x × 1xx + 1x⇒8 cos3θ = x3 + 1x3 + 3 2 cos θ⇒8 cos3θ - 6 cos θ = x3 + 1x3=24 cos3θ - 3 cos θ = x3 + 1x3⇒2 cos 3θ = x3 + 1x3 Hence, we can say that : 2cosnθ=xn+1xn 67 View Full Answer